axle problem???

[Kamaro Killer]

Ok, I don't know what's causing this, but maybe someone here will. I brought my car in to a speedometer shop to get the speedo recalibrated and I was told the left rear tire was shaking from side to side as it was up in the air. I also towed my car to a car wash (The engine is pulled right now) to clean the engine compartment and as I was towing the car there I noticed the back left tire was shaking from side to side as if the axle is bent. The other tire (back right) was fine, it didn't seem to be moving at all, so I don't know if it could be a bent axle or what? I know the back left rim is bent a little, (from an incident with a curb, from the previous owner) but even before I rotated my tires, when I had a brand new rim on the back it was still shaking. If anyone has any idea what it could be, please let me know, thanks, Keith.