99 rear end clank???

when I go over a speed bump, on the descent of the speed bump there is thump thump...almost feeling like someone placed a brick in front of a rear wheel. Sometimes when braking to below 5mph it clanks. but if coasting to under 5 mph it doesnt. Have checked U-bolts, Leaf Springs, New Shocks, Driveshafts and joints good. Any Ideas?? thanks. Josh

forgot to mention it's the 8.25 rear end.

how sagged are the rear leafs?? could be the axle hitting the bump stops.