You are the 1st person that I know of asking this question...most cars tend to have a little rear sag in stock trim. Have you verified all is stock out back?...no air shocks, helper springs, coil-overs on stock shocks, lift shackles, or tow package type springs already in place?
With the axle housing mounted on top of the leaf springs as yours is, the easiest way to lower the rear end is to add lowering blocks between the rear axle housing and leaf springs. This is done by supporting the car by the frame and letting the suspension go to full droop. Undo the 'U' bolts on each side, jack the rear end up off the springs a bit, slide in some blocks, install slightly longer 'U' bolts to accomadate the blocks, and you are done. The same blocks that are used to lower pickups would work on your car. I believe they can be found in varying thicknesses, you just have to search. Summit, 4Wheel Parts Wholesalers, etc. Good luck.