71 Challenger Convertible 340 Vibration

[71vert340]

I just want to plan my winter work on my Challenger. Here's the problem. At 40 mph, I notice a vibration and as I go faster it gets worse. I can't drive it over 60 mph now without a really bad vibration. I put less than 50 miles a year on it. It has the original drivetrain, 4 speed, 8-3/4 rear end, drum brakes. It has old Goodyear Eagle radials on it. Maybe 5000 miles on the tires that were bought around 1990. U-joints are about the same age. Original harmonic balancer. Tranny not rebuilt. About 120K miles on it. Good compression in all cylinders. Doesn't burn oil. No engine leaks. If I'm going 50 mph and push in on clutch, vibration is worse. If I put tranny in neutral at 50  mph and let engine go to idle, vibration is bad. So part of the problem seems to be tranny on back. I notice the rubber in harmonic balancer is cracked. The tranny tailshaft housing bushing has never been replaced.  Here's what I propose to do.
1. New Polyglas tires all around.
2. New u-joints and have driveshaft balanced
3. Have harmonic balancer restored.
4. Pull transmission and have the tailshaft housing bushing replaced.
Is there anything else that I may be missing?
The clutch, throw-out bearing was replaced about 20K miles ago and has never been thrashed. I do not remember replacing the pilot bushing then. The rear end has never been rebuilt.
  Any other items I should replace as long as I'm under there such as the clutch? I'm in my mid-60s and want this to be my last time. Anything I may be missing. Thanks.
Terry

[Cuda Cody]

Since it is speed related I think you can rule the engine out.  If it was RMP or engine related then it would do it at different speeds as you go through the gears.  And since the harmonic balancer is rotating at the same speed the engine is I would not start there.  The great thing about your problem is it is consistent.  Those are the easiest to find and fix.  Can you get a set of tires from another car and try those before you go spend any money?  It might be a simple easy thing to check by putting a different set of tires on it and see how it responds.  And I would only change one thing at a time so you know if you found the problem or not.  Good luck and let us know what it ends up being.

[RUNCHARGER]

I think your plan is solid. For the effort it takes, can you borrow another set of wheels and tires and go for a drive?

[7212Mopar]

If water pump going out, the fan will be wobbling and will cause vibration to the engine. Happened to me once.

[HP_Cuda]

Seems to be a tranny problem like a bearing going out.

But always start with the cheapest fixes and move to the more expensive ones.

[Shane Kelley]

I had this same issue with my Cuda when I bought it. Hell I changed the whole motor, flywheel ,clutch, gears and driveshaft. Ended up being the main shaft in the transmission. I put the car on stands and stuck a yoke in the transmission and let it run in gear. I could see a slight wobble in the yoke. Sent it to Brewers for a rebuild. They called and confirmed that the main shaft was bent. I'm not saying that's your problem. But definitely something to look at.

[Rich G.]

Well if you're not moving and slowly raise the rpm up to around 4K and have no vibration you can eliminate anything from the clutch forward. You can raise the wheels off the ground about an inch and spin the wheel to check for out of round tire or bent rim.