T-44587AC is used to support the main shaft during assembly, providing preload. Why? Once the rear half is assembled and the extension housing is bolted in place, you must roll the trans back over, bellhousing flange “up”.
During this procedure, if not preloaded, the main shaft is able to move. Often, this causes the P3 sun gear thrust bearing to fall out of place. At a minimum, this creates clearance issues costing the time spent disassembling.
If gone unnoticed, the bearing gets pinched and self-destructs on the test run, taking out costly hard parts with it.