HELP! 93 6.5 4L80E Trans issues

Hello I hope I am putting this thread in the right place, I have made a few searches and didnt come up with the info I need.

My dad has a 93 plow truck its a 2500 4x4 with 6.5 and the 4L80 trans. The rig has about 140k on the odo, and the rig had never had any previous major failures, it was owned by the gov't and they gave us the spotless maint history when we bought it. Last winter the the trans started acting up, occasionally shifting "into neutral" when it should have grabbed 4th gear. We changed the fluid and filter, which really looked decent for a gm overdrive, that had 25k since the last service. We drove it some more, and it seemed to have fixed the issue, you could tell right away that it was different.

Time went on, out plowing one night and the trans lost 3rd and 4th gears.
Took it out and sent it to a local guy who had a very good reputation with gm overdrives.

Six months later we get the unit back, and install it. It came with a wiring (adapter harness) that I wasn't really sure why, but he said it needed to be done, it was an update that these units required. I as carefully and accurately as possible depicted which colors were which, and spliced the wires acccording to the instructions. (Many of the factory ends were hardened, and stained and difficult to see the real original color, such as whites and yellows and tans, but I peeled back the looming far as possible and made sure to get the colors right.

The trans works great for about a month, and then not even working the truck, just driving on the highway, it doesnt shift to 4th gear. Check fluids, and all is good, no color, no odor, just normal all around. Called the tranny shop, no answer, phone is disconnected. Called his wife at work, she says he will call us back soon as he gets home from looking for a job..... he never calls back.

I have a ton of experience with fixing transmissions, manual and automatic, but I dont care much for the late model stuff, didnt have time to learn the little tips and tricks of the 4L80, and so I turned to this guy. Can anyone give me some tips and such related to this beast so I can fix it. I suspect its some little solenoid or something that was the issue in the first place, prior to the $4000.00 rebuild that fixed nothing.

Thanks for your help!

Cody