Do you mean shift, or when you move the shifter into a gear range, it doesn't put the truck into a gear?
Yeah, I'd go for a power flush too. If it's not a 700r4, maybe the modulator hose dried out and broke off, so it's not getting any vaccuum, but that would affect auto shifting only, I think.
The linkage is definately ok? If you crawl under it (with engine OFF, just in case it decides to engage while you're under it...) and have someone shift, can you actually see the linkage moving the lever on the tranny? Will it come out of park and into nuetral? If not, then I'd say the linkage is messed up, or maybe somehow the lever on the tranny is loose.
How'd the fluid look and smell?
Maybe water got in it and wreaked oxidation havoc while it sat...
If the linkage is ok, drop the pan and see what things look like in there. If it's gunked and watery, power flush it before running it again.
Ratch
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