Hello.
I have a chevrolet workhorse box van. Comes with a vortec v6 4.3 liter engine and 4l80e transmission, van made in 2000.
Got the vehicle from a bread company, know nothing about its service history, fixed the engine but not touched the transmission.
Symptoms were hard banging while shifting, it worked for a day, rode it for 10 miles at most, then the van was left for half a year.
Now tried to move the van, it moved forward for some 10 meters and would not go forward again, if engine revved then I could feel a feeble wanting of the wheels to rotate but nothing else, reverse was useful for bringing the van backwards into its place. Unplugging the transmission harness or removing the fuse had no effect.

So I pulled the transmission and started the disassembly. The friction packs are 0.07" in one of the pack, and 0.08" thick in some other pack, I cant remember which is which, I just tried to get some measurements to see if the friction plates are in spec. The metals look good to me.
The bushings look not bad, the only one that is pitted is on the pump assembly. Do not see any obvious signs of a catastrophe.
I tested the solenoids for Ohm values, they appear to be in spec. The plate between the valve body and the transmission housing has some check ball "damage".
Haven't checked the harness for conductivity yet. Do not see any reason to disassemble the clutch packs with pistons in them. The transmission had some black "sooth" internally but nothing overly thick or muddy.

So I do not know what is the recommended kit to get or what parts should I replace or service. I guess the separator plate with is a must. Do the bushings look ok?
What else can I troubleshoot for the problem I am experiencing?