I'd agree - check the brake interlock - that's easy. If you don't hear a 'click' from the center console area when you step on / hold down the brake pedal... that might be all you need.
If you hear the click, take a look underneath to see if the end of the shifter cable is hanging free, as that happens often (a spare one should always be on-board, as so many ppl on this site suggest.)
The hard thing to believe is that the dealer (assuming that's who has it) hasn't already had his mech look at the vehicle and rule those two things out. So you might have a bigger project on your hands. Assuming the very worst...at that point, you decide... do I throw a 'new' transmission in (note the quote marks), or just part it out and sell it?
You know how much a trans would cost, depending on which way you go - $350 or so all the way up to $2000 or more. You'd probably be able to sell the vehicle for about $4K (esp. to a private party, but how many ppl have $4K cash - that want an Envoy XL ?) Denalis fetch a little more, but we'll assume it's an SLT, since you didn't mention Denali.
The 5.3 (depending on mileage / condition) is worth $1500 by itself. All of the glass (windshield, tailgate, windows) should fetch $500 (you might have to piece them out, but $500 should be easy for all of those.
So...we're up to $2000 already, and you still have the rest of the vehicle. Front clip, interior pieces / seats...assuming you have the space to store the vehicle while it gets parted out - you'll be ahead, if you're willing to wait for the $. Since it has leather seats, it's at least an SLT.
Oh - and if you buy it and decide to part it out - contact me - I might grab most of your interior, from the door cards to the carpet, and everything in between. Seriously. I've driven to Toledo; it's not that bad for me. Although I may ask for the 'GMTNation discount'...LOL
(on edit - I may as well make an offer for the whole vehicle, and swap my good driveline in... )
