30 psi up front and 28 psi in the rear when I had 31x10.5R15s on my TJ. 24 psi on my YJ for 8 months out of the year when I lived in upstate NY. InfernoGirl has got it right IMO. Whatever pressure puts the treat on the ground flat and even. If the chalk line wears off the center first you have too much pressure. If it wears off the edges first you are too low.
'78 Scout SSII - 345, 4 speed, SOA, locked, 33's...
'06 LJ - 3" lift, 33x10.5R15's, Discos, TrueTracs, 4.56 gears...
'80 Scout II, '89 YJ, '79 Scout II, '93 YJ, '01 TJ, '00 Cherokee - all lifted, most locked and all wheeled