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In all likelihood, the source of the wobble is due to the weakening of the rear subframe mount on the unibody, induced by the narrow rounded top of the spacer, the height of the spacer, and the lack of a missing link. The unibody pad at the rear is just two thin layers of formed sheet metal with some nuts welded on. Your 2" wide spacers probably only have a contact area about 1.5" wide, and with rounded edges top and bottom. Being 6" tall creates leverage, too. Lacking a missing link allows the rear horns of the subframe to flex; the subframe may even already be cracked. The rear subframe also absorbs a lot of impact from the tires hitting the road or other objects. Of all things, the missing link is perhaps the most important part. I know you've got one ordered, but you should also consider some other lateral bracing because of the height of yours and low contact area on the spacer. The welds you've applied are not sufficient. This is what is possible without a missing link on an SFD'd truck (not mine):

Don’t be alarmed, I did the same thing recently after rebuilding my engine and having a misfire issue. I pulled off the oil cap, and there was some sort of residue there, and I freaked out, assuming the worst. In this case, what you’re seeing looks normal, at least from what I’ve seen. However, lets not just shrug it off immediately. What you’re likely seeing is the results of colder outside temperatures (assuming its winter where you are currently), short driving trips, or potentially a clogged PCV system (unlikely, but possible). You see, when your engine warms up, it dumps a lot of fuel to help it warm up faster and get the catalytic converters warmed up so emissions and oxygen sensors play nice. A warm engine is more efficient, and your crankshaft bearings take less of a beating at higher temperatures. While everything’s expanding due to heat, and oil is being whisked around, that excess fuel dumped into the cylinder can blow by the piston rings a little under idle/warming conditions. This is called blowby. The result is a small amount of gasoline mixing in with your oil. Conveniently, modern engines have a preventative measure for any damage that would cause, called the PCV (positive crankcase ventilation) valve system, which allows the pressure from the blowby, and the evaporating gasses from the fuel, to be safely vented into the intake to be burned off. The oil cap, being at the top of the engine, and being made of plastic, is typically cooler than the rest of the engine. Residual gasses from the oil and fuel blowby can condense on the oil cap, leaving behind a range of strange debris or colors (sometimes yellow, tan, or dark brown). This is completely normal, and should only really raise a flag if there is a lot of it, your valve covers and other sealed surfaces seem to be seeping oil, or you have poor engine performance/idle (this would indicate a possible faulty PCV valve). This sort of thing tends to pop up the most during colder weather, and especially in winter climates, due to short trip driving or letting the engine idle up to temperature for too long. So now that you have an idea of what the issue COULD be, it’s a lot less alarming. I really wouldn’t worry, but lets check a couple things! First of all, you said you changed the oil. By dumping the oil or checking the dipstick, a very alarming and fast indication of a blown head gasket would be a milkshake-like oil substance. Usually, it looks a lot stranger than brown sludge, so cross that off your list. Do a quick google search on blown head gasket oil, and you’ll see what I mean. It doesn’t take much for the coolant and oil to mix to make the nasty stuff, and you’d definitely notice it if it were severe enough. You also mentioned checking the radiator. While it’s less likely for the oil to make its way into the cooling system with a head gasket issue, it’s still possible, of course. Typically, the exhaust gasses are what make their way into the coolant, and while it can be hard to detect, there are texting kits for about $40 at local auto parts stores that allow you to test and see if you have exhaust gasses in your cooling system. They’re pretty simple to use, but you might only want to for peace of mind if you’re still not convinced. If you want to put some time into diagnostics further, pull your spark plugs and look at the color of the ceramic tips around the electrode. There are plenty of images online that can tell you how to “read” your spark plugs to know if you’re burning oil, running rich, lean, or have a head gasket problem. If it’s a small leak, it may not be noticeable. While you’re at it, rent a compression tester, check cylinder compression, and perform a leakdown test on the cylinders. This can be one of the fastest ways to indicate a failure of some sort, whether it’s a compression issue, valve issue, or head gasket. You could also rent a cooling system pressure tester. If you have a borescope, this can be really useful, but not everyone has one. You can perform a leakdown test according to the factory PSI specs of the vehicle. Typically, if it holds the indicated pressure (typically 13 PSI for older pathfinders), for 5 minutes or more without dropping significantly, you’re fine. Take it with a grain of salt, because a rental kit may not be 100% accurate, and could leak down even if you don’t have a leak, but it’s usually very slow. Sometimes, if you’re lucky (and unlucky I guess), coolant will come out of the damaged part of the head gasket around the heads or block, indicating failure. With a borescope, in short, you could look in each cylinder after a leakdown test to see if there is any coolant puddled up on the piston. Finally, an old trick from one of my mechanic buddies: Get a white piece of printer paper. Take the dipstick and wipe it off, dip it again for fresh oil, and put a good drop of it on the paper. Try and keep it as collected as possible and not splattered all over it. Let it sit until it soaks into the paper all of the way. It could be a while. If there is any small residue from a head gasket failure, typically it’ll sit on top of the paper and not soak in. Otherwise, the oil should soak into the paper and have very little variances in color. Obviously this is no engineering test, but it’s another quick and easy one for peace of mind. With all of this in mind, “brown sludge,” as you called it, could potentially be a different issue, sludge buildup. This comes with a range of other symptoms, however, and can be found various ways. If the previous owner took good care of her and did routine oil changes, cross that off your list. Regardless, throwing in a little Seafoam into the oil a few hundred miles before your next oil change can’t hurt, and could break up any potential sludge you do have (I usually drain about a quart of oil, dump a bottle of seafoam in, and run it like that for a day, changing the oil the next day). Good luck! Let me know if you have any questions!