I think UF will be alright. UF had a 137-game sellout streak (it was the 2nd or 3rd longest in the nation at the time), dating back to the 1989 season, just broken last year because FAU sent back 4,000 tickets like the day before the game. Despite that, UF still sold over 100% of their stadium's capacity last season.
UT hasn't exactly been filling the stadium recently either, even after reducing the number of seats:
2011
Scl-NR-Percentage of Cap-Average Per Game
UF - #13 - 100.58% of capacity - 89,061
UT - #44 - 92.37 % of capacity - 94,642
http://web1.ncaa.org/mfb/2011/Internet/attendance/FBS_CAPACITY.pdf
2010
Scl-NR-Percentage of Cap-Average Per Game
UF - #9 - 102.22% of capacity - 90,511
UT - #32 - 97.39% of capacity - 99,781
http://web1.ncaa.org/mfb/2010/Internet/attendance/FBS_CAPACITY.pdf
2009
Scl-NR-Percentage of Cap-Average Per Game
UF - #10 - 102.36% of capacity - 90,635
UT - #25 - 99.21% of capacity - 99,220
http://web1.ncaa.org/mfb/2009/Internet/attendance/FBS_CAPACITY.pdf
2008
Sch-NR-Percentage of Cap-Average Per Game
UF - #9 - 102.25% of capacity - 90,544
UT - #26 - 99.42% of capacity - 101,448
http://web1.ncaa.org/mfb/2008/Internet/attendance/FBS_CAPACITY.pdf
2007
Sch-NR-Percentage of Cap-Average Per Game
UF - #8 - 102.08% of capacity - 90,388
UT - #9 - 101.84% of capacity - 103,918
http://web1.ncaa.org/d1mfb/2007/Internet/attendance/IA_CAPACITY.pdf
2006
Sch-NR-Percentage of Cap-Average Per Game
UF - #9 - 102.10% of capacity - 90,409
UT - #10 - 101.64% of capacity - 105,789
http://web1.ncaa.org/d1mfb/2006/Internet/attendance/IA_CAPACITY.pdf
UT sells more tickets simply because their stadium is larger, but UT hasn't sold out its stadium for a season in 4 years.