I suspect I may have found the answer to this, but I have a '91 Firebird runnng 4.10 gears and at approximately 60mph/2000rpm and again at 72mph/2500rpm a "throbbing" develops, almost as if the rear axle or the driveshaft is unbalanced. At higher and lower speeds this doesn't occur.

Looking through some other posts, I found a note that due to the harmonics of steel (i.e. the driveshaft) this has been seen before by people and the apparent solution is to use an aluminum driveshaft (i.e. with different harmonics).

Am I on the right track with this?