I was chasing a transmission fluid leak for longer that I care to say. Check, and I mean really check, your trans detent spring cover… I was finding trans fluid on the underbody, rear of trans, and sometimes all the way to the underside of the rear seat area. I replaced all usual transmission seals, still had fluid leaking. I finally removed the detent cover and found it was quite warped…
For those wondering if it can be replaced with transmission in the car, yes it can, but it’s not particularly pleasant. I removed the trans brace and mounts, and was able to lower the rear of the trans enough to get a 1/4 drive 10mm socket to the bolts. you are working essentially blind, so it helps to know where the bolt heads are. You could also loosen the motor mounts to get some more downward movement of the rear of the trans, but I did not need to. A 12 point socket makes it easier to grab the bolt head as there is very little space to turn the rachet.
The bumps on the cove hinder you somewhat. Remove the cover slowly to make sure the springs stay in place, which also makes it easier to replace with the new cover. If they fall out, there are three springs, two long and one short. The short one goes in the inboard hole. Replace cover, start bolts by hand.
I don’t know if the below pic will display correctly, but it shows a straight edge against the warped cover.
Link to replacement part: