I'll offer my unsolicited thoughts on the above posts, based on my personal experience. I did last year's Rubithon Wagon Run. My vehicle was an FZJ80 with a 2-3 inch lift (OME 2850/2863), 33" BFG AT's, triple-locked, good armor, and just about 7,000 lbs fully loaded & fueled for camping. Our Wagon Run consisted of 23 rigs, if i recall correctly, so it was a big group. Of the starters, several didn't complete the run for various reasons (broken axle, punctured oil pan, side flop in an off camber area, etc...). It was a mix of 60's & 80's, I don't recall any 100 Series in the group. A few of us were on 33's, but most were on 35's.
It took us 15 hours to go the first 5 miles of the trail. We reached Buck Island Lake around 11:00 PM. We were so wiped out that the trail leader called an audible and had us spend a 2nd night at Buck Island to rest, recuperate, and repair. It was a good call.
Back to my truck - I somehow spun out the drain plug on my rear diff (back & forth on rocks, I think...). I sheared a bolt on my rear sway bar link & had to remove it. My front sway bar was pinned up, but shook loose & cut the wire to my front locker (luckily, at the end of the day as I was pulling into Rubicon Springs). On the way out, I had some minor body damage on Cadillac Hill.
I think the 35" tire requirement is to help guard against another "5 in 15" event, and I think it's well placed (at least for wagons). Maybe FJ40's or mini-trucks would have an easier time on less than 35" tires, but I don't know that from experience. Can one "make it work" on 33's? Maybe, but it depends on a lot of factors - How risk averse are you? How much body/mechanical damage can you tolerate? How much time can you allow for rock-stacking & spotting? Are you comfortable with trail repairs? Etc...
Shortly after I returned home I re-geared to 4.88's, installed a Marlin Crawler low-range t-case gear, and went back to 35's. I'm looking forward to doing the Rubithon again (though probably not this year because of scheduling issues). Would I repeat last year's experience on 33's & stock gearing in an 80 Series? Never! It just wasn't worth it for me. YMMV...
Best wishes, and I hope you don't mind my butting in.