That's Something to consider if I can't find it elsewhere. The Transmission has the Seal plus the Bellows like Dust Cover which are usually missing. I had the Driveshaft out Yesterday and did not see any Indication on the Yoke that it was rubbing on the Bushing as I think it would maybe if Bushing was worn? .

  

Never hurts to recheck the converter bolts though.

Y's guys rule!
Looking for McCullouch VS57 brackets and parts. Also looking for 28 Chrysler series 72 parts. And early Hemi parts.

MoonShadow, 292 w/McCulloch, 28 Chrysler Roadster, 354 Hemi)
Manchester, New Hampshire

Good Suggestion.Thanks

  

Have you checked wheel balance ?

No. Usually if a front Tire is bouncing it is felt through Steering. If it rear usually felt through Seat. This Vibration is felt through floor and Wheel Balance wouldn't explain the "drumming" Sound. Tire Balance would not change when shifted to Neutral at 50. Wheels are still turning.

  

What about a rear tire out of balance or out of round, or bent rim? All would change when shifted into neutral.  Do you have 2 spare tires mounted on rims that you could put on the rear just to see if it makes a difference ? What happens if you go over 50, say 65 or 70 ? I willing to bet it's a tire or wheel problem otherwise you would have a problem at all speeds.

OK. Thanks for the Suggestion. I now have a Couple of Things to check. Tire/Wheels again  and Transmission Converter Bolts.

  

Reading through this string I have a couple of ideas that might help;
1.  When a new problem develops after I work on something, I go right back there first. In this case, you may find one of the new u-joints is bad. Seems like these days you have to suspect even new stuff.
2.  Did the u-joints seat all the way into the yokes? If there's a grease fitting, is it pointed the right way or is it hitting at some point in the rotation?
3.  It's possible that at the slip yoke there's a build up of old dry grease and dirt that won't let it slide all the way in. 
4.  Sometimes new grease can seal the splines on the slip yoke and so can add pressure so the driveshaft doesn't slide forward as easily and can act like the dry grease thing too, but it goes away until you extend the driveshaft again, which happens every time you flex the suspension.
5.  Wheel bearings going bad are like a fast vibration and can sound almost like a train or like running mud/snow tires on asphalt. Especially with the rear you may not feel or hear anything with the tires off the ground.
6.  There is mention in the string about the exhaust being a possible culprit. I'd suggest wedging the pipes where they go through the frame with something that won't catch fire or melt and see if the noise changes. This isn't to "fix" it, just to see if it changes. 
7.  If while you're under it looking, the wheels are off the ground and the full weight isn't on them, you could miss seeing the problem, especially if the axel is hanging down. 
8.  Did anything fight you while taking it apart? Maybe if you had to beat on something, it got tweaked a little?
Hope this helps and you find the issue quickly and it's easy to fix. It is possible that a new issue just happened to start right after the u-joint and motor mounts were replaced, but the odds are against it. I'm supposing that there was a problem causing you to do the work in the first place. If so, did the work change the problem or create it?

Life isn't about arriving at the end in a pristine, well preserved body; It's about sliding in sideways, all clapped out, yelling "What a ride!"

Thanks Roy. I did pull the Driveshaft and verify U-Joints were moving freely and I did re Grease them. Grease did flow to all Cups. I am going to look again at the Exhaust. Vibration occurs between 20-50 mph. when shifted to neutral it clears up and disappears over 50. Because it does disappear when in neutral I do not think the Problem is Wheel/Tire/Axle related. They are still turning in Neutral. I was not having Trouble prior to having Exhaust Work done so thats what I am going to have another look at.

  

Sounds like a good place to start. Maybe just by wedging it you'll hear a change or stop it altogether.
Does the slip yoke go far enough forward?
Good Luck!

Life isn't about arriving at the end in a pristine, well preserved body; It's about sliding in sideways, all clapped out, yelling "What a ride!"