It can be done in the truck pretty easy. Valve body needs to be dropped. Your local Allison dealler will have one in stock, maybe even a used one. Call me when you get it and I'll walk you through it.
If it has the early style roll pin, I feel for yeah. Mine was a true PITA to get out as it was froze in solid. The replacement pin from ALLISON just pushes in easily though and is locked in by the valve body.