06 Envoy Denali 4x4 lost overdrive

My truck only has 37k miles on it and when I pulled onto the road I heard a loud noise and it was like I was in neutral. I made it back home by manually shifting from 1st to 2nd to 3rd. Reverse still works too. There are also no dummy lights on. Anybody have any ideas??? GM's sending a flatbed out to pick it up on Monday to take it to the dealer. Thank goodness I bought the extended warranty. I get real frustrated when I'm trying to back the company I work for when something like this happens to me....

It is internal, just sit back and let them fix it.

I did some research and it sounds like the forward sprag might have broke. I'm going to sit back and let them fix it but I was just wondering if anyone else had problems like this.