check the trailing arm and radius arm chassis end bushes and bolts
could be a sticky brake that is binding just enough to pull to one side jack each corner in turn and spin the wheel if there is more resistance on one than the other then that will show you which brake is sticking then you can have the wheel off on the offending corner and see why it's sticking and rectify it.
Back axle is lozenging.
Rebush the whole back suspension with polybushes.
Rebush it won't cost more than 30 quid if u use std bushes