I'm running 34x10.50s on 2" lift. Your problem will be with the width of the 12.50s. I believe you can take enough out of the front to clear, but be VERY cautious in the rear and hammer down all the pinch seams in the fender well. My 34x10.50s clear in the back on 15x8s with 4" backspacing, but I don't know how 33x12.50s would do with whatever wheels you choose to run.
Tips on trimming the front:
Take out as little of the inner fender as possible, to keep the mud off the engine and wiring.
I ended up cutting off the bottom 6" or so and zip-tying them to both the fender inner bracing (you'll see it once you get the foam out of the door jamb) and the frame below the A/C (on both sides, there's a hole through the frame- on the driver side it's the hole for the speedometer cable) to keep it out of the way out of the tire. Use strong zip-ties and use two on each connection. I used an existing screw hole to attach to the fender bracing and cut a hole to attach it to the fender. For the front side, just let it rub the poly inner a few times around the yard, see WHERE it's rubbing, and clear it the best you can, but don't cut out all of the angling, as that's what provides a lot of the strength. I didn't look into cutting out all of the angling and tying it up elsewhere because I didn't need that much clearance. Also, be sure to leave at least one of the bolts on the front to add to the stability. Follow those basic instructions and you'll clear the tires while maintaining the stock inner fender for maximum engine protection.
For the fender sheet metal I had to cut through the front bolt hole (out of two) on the back side of the fender and I just lopped off the corner of the front of the fender opening. No big deal.
Go buy a 6# mallet and hammer the piss out of any pinch seams or corners that piss you off.

You'll want to knock down the pinch going vertically (this one can slice the tires), as well as the one going horizontally, just to be safe.
Here is mine on its maiden voyage, stuffing the rear tire...
I hope this helps you out. Don't let anyone fool you- it can be done.