Ive had this D3b for 20 years transmission don't want to move in first gear around 100 psi ,2nd and third have 300 or more,oil isn't black,but I'm leaning towards clutch pack,took control valve apart didn't find anything
Drain the transmission oil. Is it dark or does it look/smell burned.? While you have the oil drained remove and clean the suction screen that is in the suction side of the powertrain oil lines prior to the transmission pump. Your O&M Manual will have a procedure for that. Post photos of whatever material you find in that screen.
sounds like the first gear range clutch seals