Help! th350 wont shift from 1st to 2nd

[As2rd]

Im trying to sell my truck so I can get something that suits what i need. I know waah but the truck is acting up whenever someone wants to test drive it. The last guy who was looking at it is interested in purchasing it if i get the tranny fixed. The thing is, I dunno whats going on. I replaced the fluid, changed the filter, replaced the mod, the vacuum line, and the fluid is at perfect level and is red.

I think that i let the truck sit too long, because of the weather, and the fluid isnt circulating. Anyone have an idea or solution? All input is welcome.