changing the front ifs differential gears. sorry about the mix up
ok, whole different story. Does the spare diff you have sitting out have the stub shafts already in it? If it has a flange on both ends with 6 bolts pressed through it, then yes it does. If that is the case, all you have to do is take off the 6 nuts on either flange on the diff that is still in the truck, and use a long screwdriver or pry bar to push the flange off the CV section of the axle. pull the diff, put the other one in, and reverse the process. When you put the other diff in, make sure the axle ends are above the diff, and not hanging down, it is a lot easier to get them to line up when you're bringing it down, than pushing up.
If you need to pull those stub shafts out of the diff, you've got one of two options. One is to stick a large screwdriver between the diff and the flange, and pry it out. It'll be tight, but then it'll pop loose, there is a c-clip holding it in the diff. The other option is to take off the diff cover, and find the ends of the axle shafts, and tap/push them out from the inside.
You could also take a CV, slide it over the 6 bolts, thread all 6 nuts on a few threads, and give it a good yank, essentially using the CV as some sort of slide hammer. Make sure you've got the diff held down real well though, it'd suck to pull that onto your foot
*EDIT*
This is a pic of the backside of the flange, which is supposed to be attached to the stubshaft shown below. Obviously in my case it wasn't, but just an FYI, I got that stub shaft out of there by gripping it with the visegrips, and prying against the visegrips and the housing with a screwdriver. It takes a little bit of effort, but you can pry it out.
