Well, the R50 and R52 Pathfinders are unibody chassis, so you can't use either of them. The R51 has a frame, but it's completely different than a WD21 frame and I've never heard of anyone doing that kind of swap. I would imagine it would take a lot of fab work and I'm pretty sure the R51 frame is longer.
An engine swap would be easier, but it all depends what you want to put into it. The easiest would be a VG33E swap, but that's not going to give you a whole lot more power that what you have. Next in Nissan would be a VQ-series V6 swap, which definitively has more power, but requires swapping the ECM and wiring and merging that wiring with that in your 94 model. The VQ35DE and transmission out of a 01-04 Pathfinder would probably be the easiest. The later, VQ40DE and RE5R05A trans would be a bit more difficult for several reasons. They do make engine conversion brackets for a small block Chevy for the Hardbody trucks, which should work in your Pathfinder, if you want to go with a non-Nissan option. Anything is possible where there is a will and a wallet!