I have done a couple 4000 RPM burnouts and no vibration. Probably the yolk I think.
too be sure, try a couple at 6500 that should help!!!
Oh, yea, sure!!!! Spin that imbalance out of your drive train!!!! BAD imbalance, BAD!!!! I am sure an imbalanced yolk will be the least of your worries then!!! You have a scatter shield on this?? C2, you are not............., maybe................
The vibration seems to be speed related, not rpm related. Like I said with the clutch pulled in it still does it at 50 to 60 MPH. The motor in this car is rollerized and balanced so it could do 6500 RPM smoker as long as somebody else pays for the repairs. I don't misbehave to often but if called on, I let it rip!
A constant droan or one that comes and goes? I would go with the yolk, what else really is there? A bad gear set will give a whine, howl and even if it has excessive backlash, it wouldn't/shouldn't give a driveline vibration. Besides, you confirmed that with the gear swap!! When the weather breaks, you can always take it to a shop that specializes in driveline issues. They could measure the runout on the yolk and go from there............
The last owner said it always had the vibration. He changed the gears and it was the same. It is either the pinion angle or the yolk. The axles are a long shot and back lash should be there if you let off. If you hold the speed at 55MPH it is a constant vibration. Believe me it is fustrating.
Whoa...This is a girly car??? I know how mad it can make a guy..My Rag was giving grief before I "rested" it in '02 and I finally took it out this summer and cured it...But now I can drive it..I would usually have to drive it @70mph to be fairly decent but it would get nasty till I got under 40mph! Shimming is easier than the yolk. Try that first?
Sounds dumb maybe but sometimes mysterious speed specific vibrations can be solved w/ a new set of shocks.. Try the towing w/ the d'shaft out trick and if no luck, unless the shocks are new, try a set of fronts first...especially if the car has been sitting for a while!. Good luck!.
__________________
I like real cars best...especially the REAL real ones! ----------------------------------------------------------------
Lots of things that I didn't think of came up on this post. So far I have the pinion angle ( either shim the tranny or adjust the rear end), yolk wobble, rear axel wobble( tow with drive shaft out) , gear lash, clutch, and flywheel.
I have swap a tranny, driveshaft, and tires. Any other ideas?
before you get too carried away, run the car up on jack stands, (under the rear end so it's at ride height) have a good look for excessive axle flange runout, (wheel wobble), pinion yoke runout. gear lash isn't going to cause a vibration, clutch or flywheel vibration you would feel without the car moving. brakes adjusted too tight will also give you a vibration...
One other thing to check - as simple as it seems, the engine & trans mounts. I had a similar problem a few years back; I checked everything rotating and couldn't find anything out of balance. Turns out it was a mount, weak but not so obvious that you'd see it. Baffling at the time but in the end a simple problem.
After seeing Feris buler's day off and seeing a sports car shoot off a set of jack stands while running I have been scared to do the same. It seems like a good idea to check the axle and yolk play, but what causes that problem?
that should help!!!
C2, 
, maybe................
I know how mad it can make a guy..My Rag was giving grief before I "rested" it in '02 and I finally took it out this summer and cured it...But now I can drive it..I would usually have to drive it @70mph to be fairly decent but it would get nasty till I got under 40mph! Shimming is easier than the yolk. Try that first?