At 90K I had to pull the converter out of my '00. Setting a trans slippage code, line pressures were fine, no material in the pan, oil not burnt. Clucth plate must have glazed over. Previous owner didn't tow much of anything with it.
Installed a aftermarket tow converter, installed trans-go 4L80E-HD2 valve body kit and mobil 1 synthetic fluid. Shifts much firmer, not harsh, converter comes in really firm. Towed my dodge+camper with a GCVW of about 17,000lbs and worked great. Above 60 I ran OD as long as the RPMs would stay about 2000, then I'd down shift and run 3rd.
EDIT: The 4L80E in my '91 suburban took a dump however. 4th gear mechanically failed, hard parts. Truck has 106K on it and the trans was replaced with a GM SRTA @ 88K.