Tranny Gurus: 1977 Power Wagon Won't Upshift

I'm looking for a diagnosis. The truck is a 1977 M-882 military truck 5/4 ton 4x4 (basically same as 3/4 ton) with 318/727. I drove the truck to get the Christmas tree in early December (It only gets used a few times per year) and the tranny started slipping coming home. It was low on fluid, so I added fluid. The transmission won't upshift out of first gear. Reverse works OK, as does first gear. I've dropped the pan and changed the filter and fluid, but that didn't help. I then lubed the kickdown linkage and even tried unhooking the kickdown linkage, but that didn't help either. Is my tranny fried or is it kickdown-related that can be fixed by dropping the pan again?

hook the kickdown linkage back up and hook a gauge to the governor port & see what psi you have. If Kunkel sees this he'll help you more than I can

If it won't upshift with the TP linkage disconnected it's not a linkage problem.

When the pan was off did you check/adjust the kickdown band?

No, I didn't check it. My knowledge of trannys is pretty much limited to changing the filter but I'd like to learn. What do I need to do/look for on the kickdown band?