The general rule-of-thumb is that tire width (at the sidewall, not the tread) shouldn't be more than 2 inches wider than the rim width, but people exceed that routinely. A 285 is 11.22 inches, so those tires in American off-roading parlance are equivalent to 32x11.22.
Most Jeep Cherokees (the original Cherokee, not the current Cherokee) and Comanches came with 7-inch rims. It's very common for people to run 31x10.50 tires on the 7-inch rims, and it's not unheard of to run 32x11.50s. I think you'll be fine running those tires on the 6.5-inch rims, but I would go a bit smaller when you replace them.
If I don't do anything, and someday I have to put the spare tire on, it needs to go on the front so I don't ruin the differential, right? Or else drive *really* slow if I can't do that and it's on the back. (the truck is RWD)
Does the truck have a limited slip differential? If so, then keep the two tires on the back the same size. If you don't have a limited slip, it doesn't matter whether or not the two rear tires are the same size.