i really need some help

Being I work for Nissan in the service dept. I can throw out a few ideas. First of all, make sure there is nothing bent or broken under the shifter plate at the base of the shifter. All the linkage is internal with an opening at the top of the transmission incorporating the shift gate. You can check this from inside the truck by popping up the console trim and unbolting the shift boot retainer. If this checks out OK, and the clutch master & slave are not leaking and operating properly, you may have fragged the shift fork for reverse as previously mentioned. While this is NOT common, I never rule anything out.

The Nissan drivetrains are fairly bullet-proof with the exception of maybe doing a clutch every 180,000 miles or so :D

Let us know what you find.