You said your's is a 91, have any of the suspension parts been replaced yet? What i'm thinking is possible a previous owner used to carry stuff in teh trunk over the years and the back shocks or springs are starting to wear out (and at a faster rate than the fronts?) Or it could just be that teh fronts are stiffer, i know with my setup now the front is stiffer, but with 250lb springs in the back i can hardly call that soft
But maybe check that out, just see if the back suspension is worn out?