Has your actual fuel mileage changed? Not the rate the needle falls, but the actually miles you are getting per gallon?

For the vibration, if its hard to tell while driving, another thought, is to put the rig up on 4 jack stands so all 4 wheels are off the ground, start the jeep, put it in drive and let it idle, while watching everything spin, and look for anything that looks out of balance.

Sean

99 XJ Sport 2 Door, 4.0L, 5 speed, all stock. Daily Driver
79 F150 Standard Cab Short Box, 400ci, NP435, NP205, 35" SSRs