Its possible that after the kerbing that the diff mount rubber was split, if you haven't checked i suggest you have a look . Even if it was ok , now you have stiffened the rear of the car up it will be taking more of a battering than usual and may be allowing the beam to move sideways .
I fitted an Escort Cosworth rear anti roll bar on my Sapphire racecar, sure it helped with cornering but the axle started moving around on the diff mount ! In fact i could even hear it clonking with my Crash Helmet on .
Hope this helps
Cheers