First off i guess ill include my 4x4 info:
its a 1994 s10 blazer 4dr 4x4
4.3 Vortec
Auto trans and push button 4x4(the switch is right above the lights)

So my four wheel drive isnt working. When I push the 4 high I can hear it shift as well as the light on the button stays lit and when I push 4 low I can hear it and you feel the difference along with a lit up light. With that being said I would assume that my transfercase is working in good order. Ive read the "4x4 no worky" thread along with many others and have checked a few things. The actuator under the battery is in working order(when you suck on the tube if draws in and pulls the cable to lock the axel in. However the even when the cable is pulled the it wont lock in the axel. I used a pliers and pulled it to lock it in didnt work, also I changed the cable and made sure every thing was good.
Also one more bit of info, when both front wheels are jacked off the ground if i spin one the other doesnt go the opposite direction. From what I understand they should, but maybe only if the axels locked in?

I have an entire front axel/differential assembly from a 91 s10 blazer 4x4 and was wondering, how hard it would be to swap them out? I pulled this one out but being it had no front clip, motor, or tranny I had a lot of space. I know this one locks in and everything but is there anything I should do before I take such a drastic measure? Also anything I should do to the diff while its out if I do go ahead and swap out?

Any help would be great and I would really like to have my 4x4 working.

 

First I would measure the vaccuum at the vaccuum switch located on the top of the transfer case. Then apply that directly to the vaccuum actuator via a vaccuum hand pump(can buy them at an auto parts store) If the diaphram goes all the way in then you are good. If it doesn't then you have different or possibly bigger problems.
My truck doesn't do that with the wheels either. I was told that this is how these 4x4 are desigened. So if one of the back wheels starts slipping the oppisite front grabs traction. Believe me I was flinging mud with it earlier this week this way.