My 2 cents. I've got an older truck (46) but I had some vibration issues when my rear motor mounts started getting some age on them. The rear of the engine settled a bit until it started grounding out against the frame crossmember. I know you replaced rear mounts, but you should see a bit of daylight between the bellhousing and the frame crossmember.
Also you haven't mentioned if the vibration is constant once it starts. Does it differ when under a load, floating at speed or coasting? I'm going back to your comment about being able to move the front of the drive shaft. The front and rear driveshaft bushings shouldn't allow that. If those bushings are bad, the oil seal is too. Have you noticed that your rear is getting overfull with lube. If so it's coming from the trans, down past the bad bushings/ oil seal.
Last edited by David Bush; 04/15/2015 2:24 PM.
