I don't like to use the curb weight, it may have weighed 4600# back when it was first built but chances are you won't see anything close to that number if you were to drive it up onto a scale. Modifications & equipment added over the years all add up. My old town landfill had a truck scale that was used to figure out how much you'd dumped & what you paid, I weighed one of my 78's there before heading out on a trail ride and tipped it at just a hair over 7300lbs. I was initially surprised until I started adding up all the things that it *didn't* have when it was delivered to the dealership: 460, dana 60's, 38.5's, steel rims, custom heavy duty bumpers, push bar, custom 3/8" skid plates, front & rear axle trusses, 12,000lb winch, spare tire, full roll cage (front & rear), hi-lift jack & the top chest of my roll-away all added up. Now he may not have all of that on HIS truck but chances are it's not stock and he'll bring tools & spares which I doubt he's going to carry in the tow rig (and they weigh what they weigh regardless of *which* vehicle they're in). I like to use the GVW because I think it's a more real-world number. Just running 33's on steel rims probably adds 200lbs to the truck over the stock 31's.
Towing it with no fuel isn't a bad idea, easy enough to fill it when you get where you're going.
You're right as far as the 2WD vs. 4WD tow rating, the drivetrain (engine, tranny & axle) are only capable of moving a certain ammount of mass around, if you reduce the weight of the tow vehicle than you increase the weight that you can tow by that ammount.