vibration in rear end of 930

[fredmeister]

I have a problem that I am trying to isolate and fix in my 79 930 turbo.
The car starts to vibrate around 65mph and gets worse by 70-75mph, so much that the front steering wheel is shaking.
It is definitley speed related because the frequency of shaking increases with speed, but at 90mph at least it does not shake as strongly.
My question is it possible that CV joints will cause this?
I do not see flat spots on my tires, and it looks like they have not thrown any lead balance weights. Basically the car was fine with the same tires for the last year. It just started doing this after a track day last month.
I may be able to change tires with another set of rims and eliminate that as a cause, but still feel it is something in the cv joints or possibly the brakes?
---------How would you check to see if it was a cv joint problem?
-----Is it normal for the axels to shift in and out, basically along the line of the axel axis, as the wheels turn.....I noticed this when under the car and turning the wheels.

[h20cooled7]

possible bent rim or rims? put on sturdy jack stands and have some one ease off the clutch in first gear and watch the drivetrain for bent rim or out of round tire. if the axles are that worn out you might be able to feel the play front to rear . also shake the the rear end down for play to make sure nothing is loose. is your clutch ok?

[Install425]

I had same problem once, loose axel bolts. It continued until to bolts broke. Now I check them every once and a while.