I have a 98 cherokee SE 4x4, with what i believe are 15x7 stock 5 spoke wheels. Does anyone know the offset in mm? the only accurate measuremtn i could make was from the front of the rim to the mounting surface was 2.75", by my calcuations that would mean a +20mm offset, but that sounds wrong, please help!

I don't know what the offset is, but Google is your friend:

http://www.tirerack.com/wheels/tech/techpage.jsp?techid=101

http://www.yokohamatire.com/utcustom.asp

http://www.1010tires.com/WheelOffsetCalculator.asp

You should be able to measure it....enjoy!