If you removed your drive shaft then you might want to check everything you touched to make sure everything is seated properly.
I couldn't see how a lift would make your rig vibrate.
Also I can't see the angle of drive shaft making it vibrate unless the u joints are hitting as they are turning but then it wouldn't be only between 20 and 50 mph.
Interesting, I am really looking forward to finding out what this is.
Keep us posted! smile