I just installed new rear springs in my 72 malibu. The springs were just your generic rear lift springs from JC Whitney, and some measurements afterwards are 29" to the tops of both front and rear fender wheelwells, 24" from ground to bottom of rear bumper, 16" from ground to bottom of front bumper. The springs were around 17.5" uncompressed and now are around 15" in length (installed). The shocks were hyperextended after installation so I just threw on a couple 3" extenders for now; the shocks' lengths are about 18" installed. Now the problem is that I have bad vibration in the rear end while driving. During gradual acceleration vibration peaks around 30 mph and is very low tone, then it stables out a little around 45, and becomes a problem again around 60. By 70 I don't really feel in control of the car, because the shake is side to side as well as vertical. When coasting down you get the same problems at the same speeds, and it shakes considerably when driving in reverse over 10 mph. I'm not sure how to diagnose the problem between rear u-joints, wheel bearings, alignment, or wheel balance, and I don't know anything about rear sway bars and how they fit into this. Any help in guiding me along the path to take to fix this problem would be much appreciated. Thanks,

Brent

Brent,
Sounds like the pinon angle is off due to the extra height. Had a similar problem with my old 75 Camaro.

I'm with Tino, driveline angle has been changed you need to ajust the pinon angle.