My bet is the check valve and or filter is getting plugged from converter lock-up clutch material. There is a check valve in the cooler line. The converter dumps directly into the cooler line. When the check valve gets plugged fluid through the cooler is restricted. The return line from the cooler feeds the lube circuit for the O.D. unit. It can go south quickly. y mistake I saw the 2001 post and thought this truck was an earlier model. This info may not pertain to your year.
Doug