My Challenger is lowered ion the rear (and front)... I cut some of the rubber snubber off... leaving about half. but... the the rear leafs are also relatively flat and stiff (225#/5" uncompressed arch, Flex-a-Form fiberglass mono-leafs, red Koni adjustable shocks)...
You may want to stiffen your leafs (t least get some better/stiffer shocks... QA1 adjustable, Vikings, etc).. Also, avoid loading the rear end down as well as very bumpy roads, etc.