Does the rear suspension have lowering blocks? The reason I ask is that it is not uncommon to have a ride height difference as you are describing. What works well is to modify the lowering blocks to adjust out the difference. Either shorten a tall one (preferred) or make a taller one for the low side.
It could be either the front or rear springs causing the problem. As to the sway bar, it can also pre-load a suspension. Just disconnect it on one side and go for a drive and re-measure to find out.