Take it as bad advice or not, I have been doing this for a while, but I have never followed the recommended intervals for changing oil. Especially if you're dealing with time variable. The properties of the oil didn't change over the course of 24 months if you didn't put squat on it for miles unless you have some kind of contaminant in the oil. If you're dunking the rear end in a lake because you're towing a boat, or going through low water crossings routinely, You might want to change or inspect more frequently.
For a while now, I have been simply topping off some of the fluids because it leaks from A, B, or C. There are a few exceptions where you might want to be a little more diligent, but the auto manufacturers have always been pretty liberal on how frequently you should change the fluids.
The rear differential with an LSD might be a different story and require a bit more attention if you're putting a lot of miles on it. Mine does chatter, but the rig is also 31 yearl old and has 315K miles on it and I'm pretty sure everything in there is original, or at least it was when I got it... I changed the fluid when I replaced the pinion seal 9 years ago because it was leaking. Now I have a leaking rear wheel seal and the fluid will get changed after I get that done as well because the bearing grease has probably contaminated the oil.
As mentioned, a roof top carrier isn't adding much to the load. It probably matters more if you're pushing the load capacity of the vehicle when you're driving it, rather than just the carrier alone.