Check your rear U-joint. Have someone shift from drive to reverse while you look at it (from the side of course!). Then get underneath with a screwdriver or small pry bar and see if there is any movement. If none then get your tires balanced and rotated. If still there you might have a tranny problem. I would bet it is your u-joint tho.
Ok, first off, i am a CHEVY MAN, but i am also a mechenic. I would check the U joints in the rear driveshaft, it is PROBABLY one of them, that is assuming you have more than one U joint in there. I would just drop the driveshaft and feel the u joints. The shaft is easy enough to remove. When it is totally out, put it on a bench and the joints should move very freely. If they hang up, like if the spline doesnt fall from its own weight, then replace them. Understand? I hope i explained that well, cause i can see it, but i dont know how to tell about what i see. HC also had a good point about getting the tires re-balanced if the u joints arent the cause. If its not either of those, start looking into a transmission. Hope i was of some help.