1997 5sp 4x4 extended cab F150 with 117K. Shift, and get back on the gas, I get a clunk underneath your feet. Happens 50% of the time or more. Also can make it happen, going down the road, let up off the gas, coast, then get back on the gas, you can make the clunk happen at will. I am guessing u joints?

 

Could be u joints but also check the backlash in the rear diff mine had almost .030"! Just re bearinged it and shimmed until I get new gears took most of the clunk out for now

 

U joints most likely. Check bolts from transmission to motor , sometimes they work themselves loose. Also possible transmission or motor mount bad .