transmission shifts hard when hot

[DennisDuncan]

My uncle has an 04 3500 6-speed and he lives close to Pheonix, AZ. When the transmission gets hot, it can barely be shifted in or out of gears. The clutch seems to be fully disengaging too. Town driving doesn't bother it but when he gets on the highwayfor about 40-50 miles and gets it nice and warm it gets hard to shift again. If he sits for about 30 seconds it starts to shift easier. He has tried all different kind of fluids and additives also. I've read up on some stuff on the forums but haven't really found any solution. Hope you guys can help! Thank you.