How long ago did you replace your rear shocks? They usually only last around 60k miles. Most people go way over that. Also a few company's make stiffer rear springs you could also replace them. I have also heard of people putting land-cruiser springs in them to get a bit of a lift in the rear and they don't sag as much. Your springs could also be bad if you have been riding with bad shocks for a long time. If it was me I would find springs that are 30% weight over from the stock ones, then throw some bilstein 5100's under there.