Clunks when it goes into Overdrive

Its an 97 XJ with 188K and Bone stock. She's a little rough but the tranny shifts fine until it goes into overdrive then there is a "clunk" when it shifts, there isnt any slipping. Also its an 4.0L with the AW4 auto.

Add some SeaFoam Tranny Addative. That should fee up some sticking valves...If that does not work to the tranny Doctor.. Seafoam is at NAPPA