Mickey Thompson used to make 31x13.50's....don't know if they still do. But if you are going smaller for better mileage then a shorter fatter tire is going to suck up fuel just like a taller narrower one. Is there a reason you want to stay with the 10" wide rim? Maybe sell your tires and wheels as a package and replace them with a 15x8 with some 30x9.50 or 31x10.50 tires.
'84 CJ-17: 17" stretch, locked Toy axles, 5.5" lift, FI, AX15, D300 w/twin sticks, boatsides & junk.