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I have a 1994 f-150 with the 5.0 and auto tranny, 4X4. Every time I shift from drive to reverse, it makes a loud clunk and  shakes the whole truck. If I let the tranny sit in nuetral for a seconds before shifting into reverse it is fine. WHAT COULD BE WRONG?????????????
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Sounds like a classic u-joint problem. Park the truck on a level surface, put blocks in the front and rear of one of the tires (so it doesn't move), put the transmission in neutral and crawl underneath. Using both hands, try twisting, turning, pushing, and pulling on the rear driveshaft. If there is any play, then you've got a bad ujoint.
The reason why it makes the clunk in the drive-to-reverse shifting is because you're completing reversing the torque being applied through the driveshaft. With the play that the bad ujoint has, you're hearing a clunking sound. Going from neutral to reverse (or neutral to drive) you don't hear this sound because there's no resistance against the driveshaft before you switch it into gear.
You may notice a vibration on acceleration or at various speeds with the bad ujoint, too.
A parts store will have a replacement for you. It's a pretty easy swap. Typically a retainer clip (removed with needle nose pliers) and a pair of u-bolts. When you re-tighten the u-bolts, make sure not to over-tighten them...in other words, don't pinch your cheeks while tightening.
