I would look to the valve body, check balls, spool valves and such. Make sure the spool valves move freely (I once rebuilt a 4T60-E and didn't thoroughly clean one of the valves, it had one tiny little metal shard preventing a shift into reverse). Verify that the separator plate and separator plate gaskets are correct for that exact tranny. Let me know if that does it, if not, I might be able to come up with some more ideas. Sorry I can't be more helpful, it's been a while since I've rebuilt any transmissions.