Aggressive Drivetrain Shake??

If you're having a noticeable vibration something is wrong. You might get JUST enough vibration to notice, but nothing that should be a real bother, typical old car road noise (it's not a new/modern car!!). Usually it's the rubber mounts on the ends of the transmission crossmember are worn out, and/or motor mounts. The rear axle pushes on both. There are two thrust arms, one each side of the crossmember, but they have very thin hard bushings in them. I don't think there are replacements for those available, but they don't seem to be a problem. The center bearing (on both V-8 and six -- CV and single joint, respectively, also single joint 56-61 V-8) could be worn. It rides in a rubber bushing made onto the bearing. They usually don't wear out easily, but if there had been a transmission seal leak for a long time the oil can deteriorate the rubber around the bearing. I've not seen one of those cause an issue yet, but haven't had one in a long time either. Time could be a factor...

So if the CV joint isn't "floppy" at the center and seems to be holding itself straight, check the rubber end mounts on the crossmember and the motor mounts -- replace if they look old. The end mounts should hold the crossmember about centered in the channel on the ends. The channel keeps the crossmember from falling out or lifting too high if a rubber mount fails. If the crossmember is sitting on the channel it has obviously failed. Should be at least a 1/4" gap between lower edge of crossmember and the flat of the channel at the bottom -- channel extends a little over an inch from the inside of the sill ("frame" rail).

The harmonic balancer is in good shape (I jacked it up with a piece of wood underneath to try to prevent damage). The engine idle smoothly and revs up as it should in neutral.

I decided to to try with the easiest parts first, torques down all the transmission mounts, and they look to be ok shape, but due for replacement. They're not really sagging but are all cracked up. 

The bellhousing bolts were a little bit loose, though. Torqued the ones that I could get to from underneath to 40-45 lb-ft per the manual, but could not get to the top ones. I'll order an angled wrench and try to get to those this weekend.

Took it out for a drive after going through all of this, and it did do the shake, but only intially for about 3-4 seconds, and then it never came back, seems a little bit better, might be that the bellhousing is not torqued down completely.

The weirdest part about this whole thing is that it only seems to do it after it has been sitting, and this last time that I drove it, the shake never came back even under heavy acceleration. I will tighten up the rest of the bellhousing bolts and get back with what I find.

Thanks again!