When I accelerate my truck starts to vibrate badly at about 35 to 40 mph. When I turn off the OD button It calms down greatly but not quite all the way. When I am coasting It goes very smooth until I accelerate again. Any Ideas? Thanks.

Does it only do it when you are driving? If not, then put it in neutral and rev the engine, does it shake in the same rpm range as when you are driving?