hard clunk in reverse??

sounds like a worn frt pump, some times a trans gets rebuilt, the rebuilder may inspect the old pump, throw in some new gears. sometimes they don't check the gear housing for the proper clearence. i always start mine in neutral anyway, just so the fluid will pump. if you look at your trans dipstick, you will notice, ck hot while idling in neutral. this is not uncommon in older units, just keep an eye on fluid condition. as far as the clunk, i just thought it was a normal mopar trait. i have found if you apply the brakes firmly, it will lessen this.