Check for any binding or play in the diff, shafts and bearings.
It is normal for the rear wheels to toe-in (veiwing car from above, rear wheels point in at the front), both rear wheels should toe-in the same amount. Also check suspension arms and uprights do not have excessive play.
Only other thing I could think would be a loose wheel (or missing drive pin/hex) and a unglued tyre.
If you are sure the wheels are pointing in the right direction then for some reason one wheel is getting more drive than the other.