Hi there,
alrighty, a 96 reg cab short bed with the 4.9 5speed. Only when I make a sharp right turn going backwards (wheel hard over clockwise) do I get this odd clunk, and only one clunk, not rythmic or anything. I just changed the rear-end oil, I don't think it's related but it is relative.

And before someone says it, no it is not a tool in the bed box hitting the side I checked already

 

Check your ball joints, Mine would clunk once whenever I turned while backing up or going forward. The easiest way to check is to jack up one front and wiggle top to bottom, I believe that an eight inch is the maximum allowed. Repeat for the other side.

 

It could also be the radius arm bushings.

 

mhy truck is doing the sam ething...did u figure it out

 

My 92' was doing the same thing around 270,000 miles and it was the left radius arm bushing, or sway bar bushing! Lots of names can be used. Hope it helps!

 

I'll cast vote number 3 for a worn Radius Arm bushing. I could feel the clunk through the floor board.

 

Check all the ubolts on the leaf spring. My truck makes a loud thunk at the same right turn into the driveway every time . I turn right, and as i am straighting out, THUNK... found out the u bolt was letting the spring pop from the lifting then coming back down from the turn. (if that makes any sense) anyway hope thaat helps.

 

Had the same issue. Felt like I was going over a speed bump when backing up and hard right steering. Replaced all four ball joints and the problem is gone. The two uppers were in bad shape, one of which was seized and would not move at all. Both my radius arm bushings were replaced within the trucks first 50K miles. Symptoms for both radius arm bushings and the ball joints were nearly the same.

 

Sounds like radius arm bushings

 

This was it, thanks a bunch. The other answers seemed like good ideas, but mine weren't in need of replacing yet, I just rolled over 200k and they dont look original so a previous owner did something to them