My guess is 20x11 will not fit without sticking out from under the fender lip.

I have 19x9.5 with 295/30R19 tires and they are CLOSE to the inner well. I don't see how you will stick an extra 1.5 inches in there without coming out nearly a half inch outside the fender line.

I would recommend pulling one rear wheel off and taking a measurement from the fender lip to the inside wheel well. I suspect it's just under 11 inches. or just a hair over. Either way, you must remember that the inner well is tapered and as the suspension articulates, the tire/rim combo goes up and in. Rubbing on the inside wheel well is almost guaranteed with a rim that wide.

Good luck!