I'm told it's not too difficult to replace the friction plates in the electric o/d unit. From your description, yours might be slipping.

Whatever you do, don't swap it out to replace with a 5-speed M47. Those have more problems; the M46 has a better reputation overall.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, airbox heater upgraded, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, aero front face, quad horns, tach, small clock.

My lemon of an 84 240 did the exact same thing, when you were on the gas it would disengage and when I took my foot off the pedal it would engage through every gear. IIRC in mine it was the one way clutch between the trans and the overdrive that was toast, it ended up not letting me into reverse 80% of the time. I had to turn the car off wait 10 seconds shift into reverse, start the car and get enough momentum to paralell park. Was a huge PITA, but the rest of the car was not worth saving so I just put up with it till I found another one.

I'm an expert at frontal parelell parking though...

It may be a dirty solenoid. Take it out and clean it. It is probably plugged with fine dirt. If the OD engages in reverse, you'll probably destroy it.

If that doesn't work, I have a perfect J type sitting on my garage floor which I need to sell.

Yes you can replace the OD without removing the transmission. I've done it several times. You'll want to stall the engine out in reverse, or engage the OD and let the engine coast to a stop. That releases the OD's grip on the input shaft, and lets it slide off easy. Go to the 700/900 FAQ and read some.
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john