Replace transmission (and maybe transfer)

I've an AX15+NP231 with some troubles. I'm thinking about change both for new ones (or at least fully rebuilt from Novak or SmartPartsAuto).

Does it worth to change for new models (nv3550/nsg370) or keep the current ones?

I do lot of mud/sand/water.

The NV3550 is a better trans than an AX15

The NSG, I believe (without googlefu right meow) is the 6 spd from the TJ that has the granny gear. If you can find one for a good deal I would go with that. a built ax-15 is fine. but both you mention are better units.