I have a question for any/all of you, I came to the realization about a month ago my 4wd does not work...long story short my back tires got stuck in a mudhole and when I put it into 4hi (and 4lo after that) my front tires did not spin (my husband was standing outside the vehicle and confirmed this)

So we jacked up the driver side and had driver front and rear tires off the ground, put it into 4d and into D, and only the rear tire spun. The front differential driveshaft, however, did spin. So we're guessing either the previous owner removed the internals in the front differential, or the ring gear or pinion gear is bad....is there any way to find out WHAT is wrong without tearing my truck apart? I really need to have my truck, my husbands car is having an engine swap right now and is, obviously, not driveable.

 

It sounds like your front diff is not being engaged. There is an actuator near your battery, see if the cable is being pulled in when you put it in 4wd. If it does not, pull off the vacuum line and see if you have vacuum there in 4wd. If you do not, then it is most likly the vacuum switch on top of the transfer case. Otherwise it could be that you have no vacuum to your 4wd system. Let us know what you find.

 

If it was any of the diff internals, I think it'd make some kind of noise when the truck moved...
I doubt anyone would remove the internals and such...

Most likely a vacuum leak somewhere...

 

I thought of a vaccum leak...but if that were the case, would I hear it engaging and see the front driveshaft moving?

Are you saying the front differential itself(not the transmission) has something in it that engages and disengages?

 

Yup, the Blazer's 4WD is actually controlled by the fact that the front axle needs to be engaged for 4X4 to work. Like KorDog says there is a vacuum belows located under the battery box, it gets its vacuum from a switch on the transfer case. On your year there is also a vacuum accumulator (ball) to help with vacuum pressure, if any of the lines, switch or vacuum ball are defective it will not allow the front axle to engage.

First you need to figure out if you are getting enough vacuum to the actuator under the battery, if you are then you can see if the axle will still engage by pulling on the cable going to the differential after removing it from the actuator...[you could do this on jack stands or do it on a dirt road or field also]. If the axle fails to engage then you may need to either get a new actuator or cable. Sometimes the cables will rust and fail to move so I would go to your local bicycle/motorcycle shop for some cable lube to see if you can free the cable from rust...