Gotcha. The reason I asked about 'drilling the tub' is there is no drilling of the tub. You do have to drill four additional holes in the rear cross member, not the tub. It's a pretty straightforward process, though, and I did not drop the gas tank for that. You can tell how far the drill bit would need to go to penetrate the gas tank, and you really don't go anywhere near that deep, especially if you drill the holes in steps using smaller to larger drill bits (or a step bit). I did undo the nuts that hold the gas tank skid to the cross member when it came time to mount the bumper, but did not have to lower the tank more than an inch or so, just the amount it sags once you take loose the nuts holding the skid up. That's to allow you access to the innermost holes you just drilled, in order to fish through the bolts, and tighten the nuts. Really not a big deal.
It would be much easier on an LJ, because the gas tank is further away from the rear of the Jeep, giving you room to work.
FYI, I also purchase frame tie-in brackets to tie the bumper to the frame more securely. Helps if you plan to use the hitch for anything more than a bike rack...I can post some photos if you need to see some.