1996 Chevrolet Camaro tire wear

My rear tires (both) are cupping on the inside edges. I just replaced shocks but am told springs are also bad. Could the springs be the cause or is there something else that could cause this? I'm told the rear end cannot be aligned on this car as it is a solid rear axle but I'm not feeling or hearing any vibrations that would indicate a bent axle. I also believe if it was a bent axle it wouldn't just be cupping but having hard wear evenly all the way around or am I wrong? Again shocks have been replaced but springs are weak. Please help