I cant get my spare tire down from my 2006 GMC Sierra. I think the mechanism might be stripped.

The spare tire support cable has an additional safety mechanism prevents the spare tire from falling onto the road should the cable break. This mechanism often gets filled up with road grit and won't release properly when you try and lower the spare. If your spare comes down a few inches then won't go any further then this is your problem. There should be some tin brackets included with your jack that are supposed to hold the spare tire up slightly off of the ground to allow some slack in the cable which usually [but not always] will allow the stuck mechanism to come loose. There is a more detailed description of this procedure in the owners manual. IF the spare won't move at all or the mechanism just won't release no matter what you do then you may have to have the entire spare support mechanism replaced.