I have a 1999 jeep cherokee and have recently put on a lift rustys 4.5 lift i did the transfer-case drops all of it and when i get going about 60-mph and up i get this vibration and humming seems to be coming from the rear and the car but im not sure what this could be im running the stock tires size with the lift for now till i can get bigger tires but i am unsure if any one has any suggestions or ideas why this is happening please post to help me

Your lucky its even driveable. The newer 96> XJs are really sensitive to lifting. Its unusual if you can go over 3" lift without some vibes. the cause of your vibes could be a couple things either with the front or rear axle. You seem certain that its coming from the rear so heres the reason for that.
your rear driveshaft uses standard u-joints (cardian joints) at both ends. When you lift, the working angles of the ujoints increases. When a ujoint is operated at an angle, for instance the ujoint at the front of the rear driveshaft, the driveshaft will actually change speed (velocity) slightly, four times, as it goes through its 360 degrees of rotation (its in the physics of the u-joint). The ujoint at the other end of the driveshaft is 90 degrees off of rotation from the front ujoint. That 90 degree offset cancels out the rotational speed changes from the first ujoint, ONLY, if the working angles of the two joints are the same. If they arent the same, you get a vibration.
So with standard ujoints, the t-case output shaft and the rear diff pinion shaft must be parallel to each other (equal ujoint working angles), or you get vibes. The steeper the angles, the more important it is to get these angles the same. At just 2-3" of lift, the angles arent that great and a little variation is ok but as you go up in lift, the angles become higher and if they are off just a lttle, it makes a huge difference. Dropping your t-case makes the angles lower but it also means you have to match the pinion angle to the new t-case angle. And just to throw a wrench in, your pinion angle should ideally be a couple degrees lower than your t-case output shaft angle. This is to offset the for the torque being divered to the rear tires that will pull the pinion up slightly as you are driving down the road.
All of that said, at 4.5" you may want to start saving up for a slip yoke eliminator and CV driveshaft. this will reduce your driveshaft angle and add a constant velocity (CV) joint (double cardian joint). By the way, CV is called that for a reason, it doesnt change speed as it rotates.
Wow, that was long winded and clear as mud right?

that was a very well put explanation and im glad imnot the one who gave it.

that was a very well put explanation and im glad imnot the one who gave it.

I used to have a nice picture of a cardian joint in action. Its alot easier for people to understand the physics of it in picture form, at least it was for me.

thank you now i am understanding what is happening but what if i used axle shims to change the angle or whould that not be a good idea.

thank you now i am understanding what is happening but what if i used axle shims to change the angle or whould that not be a good idea.

Shims may help. Another problem which could be responsible for vibes that I forgot to mention when running a higher lift with the stock driveshaft is the slip yoke, thats the front part of the driveshaft thats on the splined output shaft of the transfer case (should be a rubber boot over it). Climb underneath and shake the front part of the driveshaft up and down where the slip yoke is. I'll bet it moves quite a bit. This is because theres not much slip yoke engaging with the output shaft splines anymore. You can buy a longer slip yoke (from a YJ). This will increase the engagement of the output shaft splines which will stabilize it. If you are determined to fix the vibes without installing a SYE and CV shaft, you will probably have to do some trial and error with the shims. A good place to start is with an angle finder. They can be found at most home centers. An angle finder should give you a good idea of what degree shim to start with. Then find a place such as a driveshaft or spring shop, that has a good selection of shims. There are aluminum or steel, I like steel, Ive had an aluminum shim split. If you cant find any local, other options would be to contact sellers of lifts or have some made locally.
I dont know how much money you want to throw at this but a fairly inexpensive fix is to order a (hack and tap) SYE made by a company called Rubicon Express for about $100, find a junk yard XJ front driveshaft and buy a Spicer flange. You can probably have it done for less than $250. If you dont feel like running to a junk yard or chasing parts, you can just order the proper driveshaft from RE for about 250-300 more. Then you can get rid of your t-case drop and all. Youll still need shims though.