265/70R15= 29.60x10.43R15. A 31x10.5R15 is not going to be exactly that big (tire manufactures never make the actual height/width as large as they claim. So the 31x10.5 should be roughly .5" larger then the 265/70.
According to this topic http://www.ranger-forums.com/forum2/...ad.php?t=20047
XLT & XL 2wd
* 15x7 stock - 215x75x15, 235x75x15, 235x60x15, 30x9.5x15, 31x10.5x15(may need to trim)
So according to this it looks like the 265/70 would be a better fit then the 31x10.5