I have a 99' 4.0 with standard heat/ac controls. Can I place dual zone controls on this vehicle without major modifications?

only if you don't consider taking out the dash and replacing the entire hvac housing a major modification.

it may be possible. check out wjjeeps.com

you would probably be better off selling your jeep and finding one with the dual climate control (it would be less of a headache) --- on the other hand., I have dual climate control and the Shittty design has left me with no heat on the passenger side and about an $800 repair bill to fix it. maybe you should stick with what you have and be happy that you have less parts that can break