what is out on the rear....camber or toe? camber is not adjustable from the factory on all subarus, but toe is. serious off road driving (ie, dirt roadin it) will cause the toe setting to slip. on the next aligntment, ask you tech doin the alignment to make sure the toe eccentric is tourqued to specs, and afterward make an index mark to see if it is slipping. the toe eccentric is on the lower control arm where it mounts to the body, by the rear diff. if your camber is out, use a camber adjustin bolt, (such as NAPA 264-36410 to adjust. they are about 15 bucks each, cheaper than most places.
if you drive the car hard, which most of us do, you need to check the align every 5-10k miles.....usually our customers do it when the tires are rotated.....cheap insurance.