Recently my 92 T4R will not reverse unless 4 Lo is selected. You can hear and feel the transmission go into reverse, but it won't move until I actually put it in 4 Lo. It will move backwards on flat ground. but any incline and the truck won't move. Truck has 276,000 miles. Rebuilt trans has about 13,000 miles. Change fluid. No debris found in the pan. Ideas?

 

Are you saying that even in 4Lo, it will only reverse on flat ground, but not on an incline? If so, it sounds like you have a slipping reverse "band"/clutch in the transmission that simply can't transmit enough torque to move the truck unless you're geared low and on flat ground.

If it moves properly in 4Lo but not at all in 2/4Hi, then the problem is more likely a faulty shift solenoid somewhere. Check out the FSM links at the top of this forum and run some transmission diagnostics.