[uwe]

Kevin,
I would load the trunk until the suspension goes way down, and take a close look with a flashlight. A drive on lift, like those ones the muffler shops use, would be very handy for this.
When you say deflection, do you mean side to side? Your Caddy is fwd, no? I could not see why the rear tire would go this far from side to side inside the fender well? Since it has coil springs, then there must be a substantial axle control system. You might investigate bushing condition etc at the same time, and do necessary repairs, to minimize side to side movement of the rear. It will make for a tighter tow.
2" is a lot of clearance, in my opinion. My wagon has about 1/4" inside the rear fenderwell, from lip to outside of tire, and about 1/2 in the front.Never scraped, ever. But that's independent suspension. If the Caddy has a solid rear axle tube, then the wheels will go straight up inside the Fender. If it's independent, then the wheels will most likely tuck in towards the top when loaded, which provides additional clearance from tire to fender bead lip.
I suggest to investigate and observe the suspension movement first, then decide how much room you need.