I can’t help you much with the diagnosis part, but if you determine it’s time for a rebuild, I found the following information helped me.
I ordered the how to rebuild your 904 and 727 book by Tom Hand. Lots of good information on the forward part of the Transmission.
As recommended by some folks here, this video was good to watch. He is rebuilding the larger 518/46RE, But they are similar, and the same processes. Overdrive portion is identical.
This guy is working on a 42 Rh, but watch through it first Because he makes a few slip ups and hast to go back and redo things. Not major, but it might save you some time.
Measuring the overdrive unit was confusing to me until I watched this guy’s video