I did have a similar problem a few years ago before my swap
Got so had that one night coming home in the pissing rain the 2 tried to kill me by throwing me sideways and nearly ended up in centre of freeway verge
Turned out my ball joint bolts had worked loose on 1 side, after torquing up problem went away
Maybe I just got crap ball joints from sbits but they only lasted a couple if years before both rears where found to be completely shot when I did my swap