Nasty Rear End vibration in 03 Cobra, no one can help me!!!

Sounds like you just have too much slop in the various bushings for high traction tires. Also you need to take it to a better shop, rumor is that shop sucks.

What is the deal with your IRS and diff bushings, are they stock? Does your car have the incorrect IRS bolts that some cars got on the assembly line by mistake? There could be a lot of things wrong or a combination of things. You need to do some research and get under there and check stuff out.

Since one set of wheels and tires ride fine and the other set does not, you might check that the wheels and/or tires are out of round - I have personally seen this. To check, put the back end up on jackstands. Using a separate jackstand, put a pointed object, like a pen, on the jackstand right at the wheel lip, as close as you can get. Now, spin the tire (or start it and put the car in first gear at idle) and you can see if the wheel is out of round. Next, same thing for the tire - put the pen right at the outer edge of the tire.

If the wheels/tires check out OK, I would suspect your differential bushings are shot. I had to re-seal my pumpkin last year and I was amazed at how soft those rubber bushings are, especially the rear one. That made for a lot of rear end movement. I replaced mine with poly and it made all the difference as far as movement and "clunk." With your HP levels, I would suspect aluminum would be a better choice - maybe Bruce's FTBR complete set for the IRS.

It could be some other suspension pieces but it would seem they would be bad with either set of wheels/tires on.

Good luck, hope you figure it out.

do a solid axle swap and be done with it > I drove a supercharged 01 or 02 before I bought my 98 and HATED the IRS rear > 03/04 are bad azz but I will never own one unless it has a solid swap >

Hell no do not do a ****ing solid axle swap... That will destroy your resale value.

problem

so is your problem a viberation or wheel hop?

the front sub frame bolts should have a 21 mm nut...if yours is an 18mm its wrong....

if wheel hop is your problem than ftb bushing kit is needed

whats normall when stock tires go back on?

different tire tread and sizes mix matched will make a car drive wierd also...

Is it constant or only under acceleration?