I can see the sides to this as I am dealing with this right now, my spare used to be under the truck but It ruins my already bad departure angle in addition to constantly getting pulled off the bead from contact, most notably this weekend at Pismo. I then moved it to my roof rack which is awesome but at the same time adds 70+lbs of weight to the roof making it unstable and had I not put it under my truck last weekend for the drive it would have made me roll on the dunes.
I just picked up a rear swing out bumper and honestly I think that its the best of both worlds, lower center of gravity, keeps the bead clean and is easily accessible with few tools.
heres a pro con list
OEM spare position
pros: out of the way, lowest center of gravity, clears 35s
cons: ruins departure angle, can get pulled off the bead with contact with ground, not quickly accessible chain can bind and sieze
roof rack
pros: out of the way, no chance of damage to spare from rocks
cons: raises Center of gravity considerably, pain in the butt to get a 70+lb tire up that high on a lift kit, drag from wind at highway speeds
swingout
pros: keeps center of gravity lower than roof, easily removable
cons: blocks some rear visibility, gets in the way of tailgate/hatch
just my 2 cents, and OP PM me if you want a spare tire mount for a roof rack, im not going to be using mine anymore but if you want it ill sell it cheap to ya.