1999 E320 4Matic Wagon 4-5 Transmission Shift Rough, Delayed

[RLGDiesel]

At 143,000 miles, the 722.6 transmission is hesitating in the uppermost gear change (4-5 or 5-overdrive?). The problem is consistent and no MIL is illuminated. Acceleration is still good, and the transmission feels strong through all the other gears with no slips or flares. At the last gear change, however, the RPMs drop down to 2000 and I feel a second more delayed clunk to around 1800 about a second later. There seems to be some subtle clunkiness when I let off the accelerator at the changing point. The fluid level seems OK at 80 C. What else should I check? Thanks!