swap both rear wheels with two known good wheels. If the problem goes away, then swap the each of the old wheels back, one at time. If the problem resurfaces, its that wheel. If it still shows problems after swap the rear wheels, put it on a rack and take a detailed look at the axel tubes.
As solid as the case and tubes appear, it is possible to bend the tube and/or crack a weld. Then look at the link arms for bends. Last, its a unibody, according to how hard and what angle he hit it could be anything. have them do a 4-wheel alignment check.
I used to beileve 4 wheel alignment check was only needed on cars with independent rear suspension. I've since changed my mind.