Yup. i'm in agreement.1clean5.8 said:could be a U joint or a pinion nut.
When you put it into reverse does it take a second to get the "clunk". if so thats just how long it takes everything to torque up. in a nice flat parking lot, put your truck in N. start it, then shift it into R with your foot off the break...see if you start moving instead of getting a "clunk". That'll prove you're in R but other stuff is twisting up because its loose (mounts, joints etc)