1994 X ( Auto). Gear Selector in Reverse and The Truck Doesn't Move. Why?

[OVERKILL700HP]

This has happened in warm and cold weather.
I'll release the parking break, move the shifter from Park to Reverse and the truck doesn't move at all.
Actually, the truck doesn't move no matter what gear I'm in occasiaonally. Sometimes moving the shifter through all the gears makes a difference.
Any idea what could be causing this?
All information appreciated.

 

[snoborder42]

This might help, occasionally mine will do this even though i have a 5speed. Sometimes the rear brake pads stick to the drums. They actually get rusted in place. All i do is give it some gas and i hear i loud thunk and then it starts moving just like normal.

 

[TPLYNCH]

Search for Low/Reverse Servo - very common problem on the A4LD transmissions

 

[falls guy]

For one make sure your tranni fluid is topped off, next I'd check to see that you don't have a leak or a pinched tranny line causing NO line pressure. You could also try changing your filter and fluid. Then you'll really see what shape you're in by what's in the pan. Maybe the filter is so plugged that you are losing pressure. Other than that I don't know what to say like tplynch said, your low reverse/ servo is probably your only other bet other than the cluches being smoked and in need of a rebuild.Check your fluid color and smell It should be red. If it is brown, and smells nasty, you probably need a rebuild.