'88 IROC, 5.0 TPI, 700R4, 75000 miles, no mods

I've read about 250+ posts and can't seem to find anyone with this problem. All gears shift fine except into OD. It WILL shift into OD but only after you aggressively accelerate past (roughly) 65-70mph and let off the gas. Once it gets into OD, it works fine as long as the engine is pulling. When I'm coasting down a hill, it will remain in OD until I slow down to below (roughly) 65mph, which causes it to downshift to 3rd. No slipping is noticed, as I have to drive that same hill (up) and OD holds. It just doesn't want to shift.

I hadn't done anything to it before it started this, and since it has, I've changed the (trans) oil and filter....no fix. Oil looked good and there was no metal in the pan. If it has to be rebuilt, that opportunity is coming up real fast, but I have other things I could do with that time if I can keep from having to rebuild.

Thanks for the help.

 

Anyone...anyone? Guess no one has had this problem. If I figure it out, I'll post the solution.