Vibration at highway speed could be a couple things. I'd start with the basics and get your Jeep to a tire/wheel shop and have them throw them onto the balance machine. Watch the tires and see what kind of 'hop' they have when spinning, if its really bad have them pop the bead off the wheel and spin the tire 180*, re-inflate and recheck to see if it took the hop away/made it better/made it worse. If its worse then just put it back to where it was and balance the tires. Make sure you have them pull the weight off the wheel before they spin it the first time. Also if the tire has no hop but it is calling for a significant amount of weight (7oz+) you should mark where it wants the weight and spin the tire 180* so it requires less weight to balance out.
What kind of tires do you have, and what size?
How many miles are on them, and how often did you rotate them?
Did you cross rotate them or just front to back?
Is the tread on the tires 'chopped' at all from bad alignment & suspension?
What is your suspension looking like (how many miles on the shocks, are they leaking at all)?
Is your Jeep properly aligned? (are your tie rod ends and ball joints good still?)
Does the steering wheel shake at all once you start to feel the vibration?
Are you certain the vibration is actually coming from the rear differential?
Does the entire vehicle vibrate or can you specifically feel its coming from the rear area?
Can you hear anything that doesn't sound right when you're going down the highway, or any other speed?
Lets start there and see if I can't help you figure it out without actually seeing the Jeep. I'm in Round Rock by the way, so I guess if you can't get it figured out I'd be willing to meet up with you and go for a ride to see if I can't help diagnose it in person. I'm not a master mechanic, though I am ASE certified and turned wrenches for a couple years before a guy ran a red light and t-boned my car, causing me to need two spinal surgeries and not being able to wrench anymore.
2000 TJ - 2.5l 5-speed 4.10: Airaid Intake, PowerAid TB Spacer, Banks header, Magnaflow Street Series CB & Performance Cat, Jet 91 Tune, Rancho RS9000XL, Dynatrac Pro Series covers, Smittybilt SRC front & rear, ProComp Xterrain 33x12.5r15 on Eagle Alloys, Power Slot rotors & Hawk pads, Rhino lined interior, etc...