lift
sye = slip yoke eliminator
basically a new output yoke for the transfer case, it also requires you to get a CV driveshaft to work with it. once you go over 4" on a TJ you will get bad vibes because the rear driveshaft is so short, the SYE and CV driveshaft eliminate this. The SYe is about $200 and the shaft about $200.
some people get around this by installing a transfer case drop and a motor mount lift instead ($100) to reduce the driveline angle. but the SYE/CV shaft is really the way to go.
are you going to wheel it? what size tires do you want to run?
it might be more cost effective to run a 2" body lift and a 2" coil spring spacer (this way you would not have to deal with the drive shaft issues)
gas
you want to increase gas mileage, but you are installing a lift??
your MPG should be around 20hwy/15 city minimum with stock tires. Obviously something else is going on inside the engine, and a stupid chip is not going to do anything. I would check/replace the following.. in this order
air filter + clean throttle body
02 sensor (upsteam one)
spark plugs
spark plugs wires + dist/coil
like Southern said, go to
www.jeepsunlimited.com