You have an open rear end if one wheel spins opposite direction. If it had a locker that was actuating they would both spin the same direction.
If it is 4 wheel drive engage front axle and see if it will move that way. That will at least let you know if you have power coming out of the front yoke on transfer case.
Next I would drop your rear driveshaft, Start the vehicle and put it in gear with e brake set and front axle in neutral, get out and see if rear yoke on T-case is spinning, have somebody rev engine and see if speed changes. This will tell you if you have normal power out of rear yoke of transfer case.
shut truck off and put rear on jack stands and spin each of the rear tires and see if yoke on rear pinion moves or spin pinion and see if wheel moves. Doing these things will at least let you know if it is in rear end or T-case. My money is on the rear end.