4 Wheel Drive/SUVs: 96 Blazer differential problems, differential problems, front wheel drive

Question
I've been experiencing, for quite some time now, a knocking in the front end when starting from a complete stop. It's gotten worse recently, and it's getting almost impossible to avoid (usually if I start slowly I can get going without the knocking). I took the car to a shop and they said the problem was in the front differential--could be a need for new bearings, could be a need for a whole new differential. Either way, the repairs are a bit too costly for me. One of the mechanics asked me if my 4wd was engaged; I thought  my truck was 2wd. We assumed that it's basically all-wheel drive as there's no way I know of to disengage the front wheel drive. Is there a way to do that, or, through removing the front differential among other things, simply converting the truck to 2wd?

Answer
there is no way to convert it back to 2 wd without it costing more than just repairing the problem. To my knowledge there wasn't any all wheel drive blazers in 96 so there has to be a selector somwhere for it. on the dash to the left of the center air register is there a group og three buttons? they may have little orange lights on them as well...or at least one of them?