2003 Silverado 4L60e Trans slips upshift 2nd to 3rd

Truck has 222000 miles on it. With gentle acceleration not noticeable, with maybe half throttle rpms increase for say one second or two and then shifts smoothly into third, However at full throttle with the shift coming at 55 mph or so, it shifts perfectly with no slip. It also acts the same manually. I had asked my mechanic about changing the fluid and if it could fix the problem, he said, "well it could help or hurt it." After thinking about it, we decided to change the fluid. He did a full flush and cleaned out the pan which had lots of sludge in it but no pieces of metal in it that would seem to indicate a major internal problem. It doesn't seem to have made it much worse if at all, but did not fix it. A friend who has some knowledge thinks it could be the torque converter. I am wary of AAMCO, and I would appreciate any insight that could help me make the correct decisions about getting it repaired.

By the evidence you presented it is not likely the converter. It is more likely the pressure control solenoid (PSC) or worn clutch plates/steels in the direct clutch, or possibly a leaking apply piston. The best way to diagnose this is to connect a pressure gauge to the line pressure test port on the left side of the case and drive the vehicle.