First, welcome to our little corner of the world. And I like the handle you chose (although I might have chosen "dodgetransmissionswhimperanddie") LOL
So... what to do? The answer will depend on some variables. To wit...
- Another vehicle available while you work on this, or is this your only whip?
- The first thing I'd do is check in where you got the part. See if the rest of their stock has the same issue (you may need to bring something to measure the height of the booster on both units.
I'll guess that this is probably the same on their other units, as well.
If so, then the next question is...
- Any other in-town resource (AAP, AZ, etc.) ? See if you can find a different brand, with a different plunger / rod (AAP sells Wearever, for example, and the rod looks a little different on their page, vs. what I saw on O'Really's)
If they're the only game in town... then it's time to turn online. Amazon Prime Member? Rock Auto customer? RA only shows remans (Cardone looks to be the best of the lot, and I'd pick another brand, if they had a better one).
Amazon looks to have *only* the Cardone reman, for a 2008 (but their search engine doesn't always get 'all' or the 'right' parts for a particular vehicle - I thought I found an ACDelco (reman) for you, but when I brought up the part, it said it *didn't* fit.
Sometimes searching on another 'sister' vehicle can help. As I found, the cross-reference for your part covers the 2006-2008 models, for TB / Envoy / Rainier
15267004 appears to be the GM part number for this; an alternate is 19286813 (but I think the first one is the better fit.) Some good news - Amazon has 12 sources for this part in the US. But every one of them are *used* (auto recycling storefronts). Prices range from $75 to about $115 or so, and you can get the list
HERE
Amazon Canada has one NEW one... at a GM dealer. In the US. Go figure...
General Motors 15267004, Power Brake Booster in Power Brake Systems.
www.amazon.ca
Now, I don't suggest you order the above one, for $264 CDN, plus shipping... but the US dealer is listed as "Jim Butler Chevy Power House". Amazon didn't have any more info on the seller, but I used my Google-Fu, and found a Jim Butler Chevrolet in Fenton, MO.
HERE's the link to their parts order form (it's an inquiry; let them know you saw this part on Amazon's CA site and you want to know if they have it and will sell to you from the dealership). Or you can call them, if you prefer that.
Note that the US Amazon does NOT list this part or ol' Jim's Amazon storefront - so it won't help you to try and order from them.
There are a few other sources that might have the part; search on that part number and 'GM' in Google, and see what you come up with. But you're correct - it's 'officially' discontinued. Usually, there's a 'supersede' #, but I didn't see one. That, or Cardone 5474832 seem to be best options. Here's a snap of Rock's x-ref for the Cardone part:
View attachment 99205
Another source for you could be someplace like car-part.com -- since you may be forced to source a used part, you may as well have a site that specializes in used (junkyard) parts look for you. There are a ton available; if I knew where you were located in NC, I'd enter a zip code and get the results. But I have to leave something for you to do, right?
All of this took me a little over an hour. It was one of the more difficult searches I've done for parts. But, hopefully, this gives you some insight on 'options'.
Finally... if NOTHING above proves fruitful... you might be forced to modify the rod, modify the pedal, or substitute another part (maybe from an earlier MY range on the GMT360s, perhaps).
Good luck!
Now, if *none* of the above works for you, then you're looking at perhaps modifying the part, or the pedal.