in the bed. i just got a full size spare for mine a few weeks ago from a buddy. he works at discount tire and got me a 35" BFG for free. used of course, but plenty of tread for a spare.
Where's it hitting under the bed? Some guys had to mod the exhaust heat shield and could get a 35" to fit
get rid of your tailgate and make one that holds your spare tire and a high lift jack like the pre-runner guys do it