What year is your jeep? Newer TJ's (03-06) where the vapor canister is located in the rear are prone to having the flex line rub on the body and developed a leak. I have repaired several of these.
Gas caps are always an issue on jeeps, the o-ring gets hard and brittle with age and doesn't seal.
Make sure you get a factory replacement cap. I have seen aftermarket caps cause problems.
Then there is also the possibility of a bad vent soleniod, purge solenoid or vacuum line.
2000 TJ 5.5 Long arm, lots of extras. R.I.P.
2005 LJ rubi 35's Clayton, lots of extras