Going back a few years since I rebuilt my D and J-type overdrives, but if i recall, the OD pump is making pressure whenever the gearbox output shaft is turning.
When you activate the solenoid, that pressure is bypassed through a valve and the overdrive is engaged.

The scary part of your situation is that even with the vehicle moving in reverse, the pump will still make pressure.
And you DON'T want the OD to engage in reverse, period! It will destroy a gear inside the OD that is intended to only move in a forward direction.

My suspicion is that the seals are gone on the valve.
Pretty sure you can access the valve and seals by removing the plate on the bottom of the gearbox, though you may need a special tool to remove a couple plugs.