The first thing that comes to mind is the rear subframe bushings. Also check the rear control arms for damage or rust issues. It happened to me. You can also check for straight tracking by driving through a puddle then observe your tire prints on dry pavement. Wheel obviously must be kept as straight as possible.. Paul.

__________________
1984 300CD red/blk 290,000 partial resto
1980 240D 4 sp Brn/palomino gone
1997 Mazda Miata STO edition wife's baby
1988 Lincoln Mark VII LSC next resto
1986 Subaru GL wagon daily
1993 Isuzu pickup field beater
2008 Honda Civic EX-L 5sp wife's new car
1978 Honda XL350
1974 Honda XL70 my Calif. ride in HS