I just had the front control arms on my M3 replaced at 170K miles (aligned also, of course). The car has Bilstein shocks - the rears and their bushings being replaced 30K miles ago - now shakes a bit over 80 MPH. The installer says that is because my tires have little wear left (2000 miles or so). I will be installing new tires this Summer anyway but does this response sound correct? Mark Keckeisen 310.880.0352
Are you getting a steering wheel wobble &/or feeling the shake in the steering wheel? I don't know if worn control arm bushings might hide imbalanced wheels or uneven tire wear, or not. Is the car shaking, or is it just a steering shake? Did you replace part #5 here (the control-arm 'wishbone' bushing)? http://www.realoem.com/bmw/enUS/showparts?id=CD93-USA-08-1997-E36-BMW-M3&diagId=31_0242 Other parts to consider - tie rods, steering rack, the flexible coupler on the steering column, maybe the strut-tower bearing/mount. I'd think most shops replacing control arms would at least take a look at those in the course of replacing control arms since they're right there anyway.
Not really a shake in the steering wheel, more of a wobble, modest difference that only someone who drives fast would notice much.
If its in the seat then something in the rear is causing the vibration. Could be your tires if they are chopped badly or unbalanced.