On my TJ I have a Warn rear tyre carrier, fitted to Warn rear bumper.
You can remove the studs from the stock tyre carrier, then fit longer studs and some penny washers to move the larger tyre away from the tailgate. You can also fit a wheel spacer, which will do the same thing. I wouldn't suggest going larger than 33" using this method, as it'll put strain on the tailgate hinges.
For an XJ, lay it down flat in the boot and strap it down, or fit to roof bars using a ladder lock.