1990 D3c series2 Drive issue

Greeting, thanks for fielding my questions. 1990 D3C Series 2 LGP dozer 4850 hours “old school Stick” controls. Purchased used 4 months ago. Operated fine without any issues for approximately 4 hours after purchase doing MINOR dirt work setting up a new manufactured home. Parked for one month. Started fine but now will not move. When put into forward or reverse gear, machine moves only a fraction, only enough to “engage” the tracks. Both tracks function similarl. Transmission Oil is clean and not burned smell, within oil level range. Changed transmission filter, no change in function. Bled off the top of the transmission filter housing, no change. all drive and brake linkages are unworn and functioning properly. Bevel gear oil was low added 1 gallon, 3.4 gallon capacity. Appreciate your feedback. Bill