it's possible its the fluid, but I would look at that as a last possibility. my first check would be a gummed up solenoid on the OD, defective OD trigger (do you have trouble engaging it as well sometimes?), or defective relay.

I would imagine taht if the fluid was really bad you wouldn't be able to engage it at all, let alone have trouble disengaging. There's always the degraded seals fix for the pistons, but this would display different symptoms (falling out of gear while engaged, etc.) with fluid leaking past the pistons.

-rt