I have a 1992 ford f250 diesel when I am accelerating or coasting even with the engine off I get a strong chattering from underneath truck. The chattering is not in the front end, no steering wheel movement, what else could it be??

It could be several things. One that come to mind is a driveshaft universal joint starting to fail. Or a broken transmission or engine mount? Jack up the rear of the vehicle and support the vehicle with jack stands. Rotate the rear wheels and see if you can duplicate the chatter or noise. Also inspect the drive axle joints (if your vehicle is 4x4). Not sure how far you have taken to diagnose this issue, but these are areas to consider.