Sometimes there's no mod available that will prevent you from getting stuck in mud. Before driving over difficult terrain, especially mud (which is usually a lot deeper than you expect), You need to decide the risk of getting stuck and have an action plan for getting yourself unstuck if you go for it and don't make it.
That being said, one of the first things any 'wheeler should get (even before tires and a lift!) is a 30,000 lb. tow strap and a pair of D-ring shackles. You can get D-ring shackles at most hardware stores, near where they sell chain, or you can buy them online as well.
30,000 lb. tow straps
D-ring shackles
Put one loop of the tow strap through the D-ring, and put the D-ring on one of the tow hooks on the front of the Pathfinder. If you have a trailer hitch, insert the loop of the tow strap through the receiver, then secure it in place with the receiver pin. Alternatively, you can use one of these.
If mud is your favorite type of terrain, you will probably not want to get AT tires, but MT tires, such as Bridgestone Dueler MT, BFG MT, Interco TRXUS or something that has a more agressive tread pattern. AT tires don't do so well in mud.
You'll also probably want to get yourself a good pressure washer so you can clean off the underbelly after you're through.
Another thing that you'll find will help your truck in most other types of terrain is reducing the air pressure in the tires to 18psi. Of course, if you do this you will need some way of reinflating the tires for extended highway driving. A cheap cigarette-lighter-powered air compressor is an affordable introductory solution.
Anyway, getting back to your question about manual hubs. Your truck comes with full-time hubs or "drive flanges" that set the entire front drivetrain in motion whenever the truck is moving, no matter what mode the transfer case is in. This is what allows you to "shift on the fly" into 4WD at speeds up to 50mph. (Mechanically, it doesn't matter what speed you're going, but you probably shouldn't be going over 50mph if you're feeling the need to shift into 4WD anyway.) Although the front drivetrain always turns, it basically "freewheels" unless 4WD is engaged. Naturally, a constantly-spinning, but unpowered drivetrain will negatively influence fuel economy, too.
Aside from potential fuel economy losses, the problem with drive flanges is that they contribute to premature wear of the CV boot, especially when a lift is installed, since the CV joint angles increase. This is why many people install manual hubs as part of a lift.
With manual hubs, you basically have two modes: locked and unlocked. When the hubs are locked, they behave just like drive flanges. When unlocked, the wheels are mechanically disengaged from the axleshafts. In other words, wheel rotation can now occur independent of drivetrain rotation.
Another advantage of manual hubs is drivetrain failure. I recently snapped a front CV axle while off-roading in Utah. I was able to unlock the hubs and continue driving without causing further damage. In fact, I drove over 1200 miles back home with a broken CV axle. This could not have been done without manual hubs.
Installing hubs with the lift is definitely optional, though. I don't think I installed manual hubs in my truck until over a year after it was lifted, and even then I did so only in hopes of increasing fuel economy. The other advantages became apparent afterwards, and besides, I didn't get as much fuel economy benefit as I had hoped anyway.