I just bought a 91 pathfinder se and i have trouble with the locks.

When the lock button is pushed all the doors lock except the driver's side rear door. That one needs help by hand. Then when you put the key in the drivers door to unlock, the lock turns yet nothing happens. When you pull up on the lock all doors unlock.

Sensor issue?

Any help is appreciated.

It aint no sensor issue , it is like you have jammed the lock metal some where in the key lock zone , that simple procedure involves just lubricating the metal peice.
If you have some auto lock functionality , then try re programming the chipset.