OK what color is the fluid now?? Sorry, but you must check the color before attempting anything else. The best way without loosing a lot of fluid is remove the square headed plug on the bottom below the solenoid.

If the fluid is grey in color or has streaks of grey, kiss your OD goodbye. Cheaper to get a used OD instead of rebuild it. The grey comes from the insides of the aluminum cone clutch as it self destructed against the internals of the OD.

OD uses unidirectional (Sprague) only in first four. Cone clutch is for OD AND reverse including engine braking.

Running the OD out of fluid would severly harm the bearing that carries the cone clutch. Hence the harmonic noise as it heated up and probably self destructed from lack of lube. Since that bearing locates and transfers pressure from springs to keep the cone clutch on the output bell, no bearing applying pressure allows the cone to slip and not drive in reverse or transfer engine braking, hence the engine having to come up to driveshaft speed when reaccelerating after slowing down.

You will be able to go forward forever, just not back up until the OD is replaced.

Duane