thats what a "jeep tech said"?
two instances could be at fault, if the T-case was lowered due to the lift, the body might be interfering with the shifter physically getting into 5th. seen it on a couple TJ's, but not that common.
second would be to re-gear the axles, as far as I know, the only interchangable transmission gear is the first gear sprag and you swap it with a euro spec Toyota gear and get a better first gear ratio. I have read that it works in a AX 15, not sure about the AX 5 or nv 3300 or 3500 or what ever the newer TJ transmission is.
I think most if not all 5th gear 5 spd transmissions are OD gears, so they will all be a similar ratio.