Pulsating vibration while cruising??

I had a similar problem, over 2000 rpm car felt like it would shake apart or if pulling a grade hard it would do some funky shaking. Found out previous owner rebuilt Torque Converter and didn't have the external wight placed correctly. New Torque converter and built new drivline as the old one was crap. After that Smooooth ride. In park didn't notice it shake, but then again I didn't rev the heck out of it either.