Once you start modifying a Jeep, "cheap and easy" sometimes goes out the window.

First off, I am never in favor of lowering the t-case. That's a crappy easy response that partially defeats the purpose of a lift to start out with. It reduces certerline ground clearance. It changes the angle of the engine and oiling galleys, and a host of other possible things. Lowering the t-case itself can cause or aggravate vibrations if the t-case mounts or engine mounts are a litte worn.

The best way is to fix the driveline itself.

Fisrt off, I would carefully check the u-joints on the rear driveshaft for any looseness at all. Sometimes, one of the rear u-joints can be a litle loose, but we never notice it if the driveshaft is operating at OEM specs. Then, when we lift the vehicle, and run the driveshaft at the limit of or exceeding OEM specs, the loose u-joints manifest themselves as driveshaft vibration.

If the rear driveshaft has a regular u-joint at the front of it, then get it modified so that there is a CV joint on the front end. Then rotate the rear axle so that the driveshaft is pointing right at the output of the t-case, within 1 or 2 degrees. I have never seen that method fail to cure rear driveline vibrations.