"So......if I found a set of new REAL 30"x9.50"r15's ......it seems like they would fit without trouble........right?"
Yes, they should fit.
"..........and is the 31x10.5 really a 31" tire?"
I think the 31x10.5 BFG's are actually 30.3" tall and just over 9" wide. My 265/75/16 BFG tires are supposed to be 10.4" wide, but they only measure 8.625" actual width contacting the road (not total width). Also keep in mind the tire will be taller or shorter based on the rim width. An 8" rim will cause the 31's to be a little shorter than a 7" rim, but they'll feel a more stable on the road. I can't say as I've seen a true 30x9.5 tire, but I'm sure they exist. But what should it matter what the tire size is acutally called, as long as it works well and doesn't cause problems.
I also have the 8" wide rims, and while the jeep is plenty stable on the road, the rims take quite a beating off-road, so be careful. We also got the 30x9.5's as a package on our Sport, with the HPG shocks, one of which is shot. I like the style of the new rims vs the old though, very sharp.
BTW, the TJ went back to the garage last night, radiator and upper tank are having a disagreement and starting to separate. Figured it was a good time to have the cassette replaced as the tape deck stopped working. Thank goodness for the extended 6/100K warranty.
JEEPN
'97 TJ Sport
'81 CJ-8 Scrambled!
'71 Commando SC-1
'51 CJ-3A
'47 CJ-2A