A tire that's out-of-round, or has a flat spot on it, can make an ungodly sound on the rear. You may want to check the tire for uneven wear before you go further. My 95 Contour became a whole different critter when I switched from a square tire to a round one.
95 Contour Zetec, ATX (wrecked, sitting in the barn)
95 Mystique Zetec, MTX
2000 Taurus SEL 3.0L Duratec
1994 Crown Victoria LX 4.6 V8
1993 Chevy Silverado 4.3 V6
| 
