Did it on mine. I was able to do it on the truck, but had to remove the fan shroud to get the sector shaft completely out. Remove the pitman arm, remove the 4 bolts that hold the top cover on, carefully push the sector shaft up from the bottom (where the pitman arm nut fastens). Take the lock nut off the top of the sector shaft where it goes through the cover, put the 2wd sector shaft in and replace the nut and put it back together. If you read through the GM service manual, it will tell you how tight to torque the lock nut on the sector shaft that protrudes through the cover. You have to tighten it until you get the proper torque turning the input shaft. I took a guess while turning the steering shaft, that would be the only reason to pull the box. You would need an inch / pound torque wrench and an adaptor to fit the input shaft on the box.
Turn the steering shaft with the old sector shaft in the box and the pitman arm off and try to match the same tension when setting up the 2wd sector shaft. Kinda 1/2 ass, but nothing flew apart ... yet! /forums/images/graemlins/smile.gif
Good Luck,
Eric M.