Just purchased a 91 F150 and if travelling at less than 60 miles an hour and you suddenly remove your foot from the gas you get a thud thud noise from the rear of the truck. Any ideas???

 

Welcome to FTE! Since the truck is new to you it would be a good idea to go through and give it an inspection. For the clunk I would start with U-joints and check to make sure everything related is in good working order. Check all of the fluids as well. Especially the rear differential fluid. It would be a good idea to give everything a run through. Check the brakes an so on and so forth. Make sure you didn't wind up with somebody's mechanical nightmare.