The vibration is definitely a common after-effect from installing a suspension lift like your 2.5" from Rancho. It's caused by the u-joints on the rear driveshaft vibrating due to the excessive angle they are now working into with the rear driveshaft's now steeper angle.
There are possible three fixes depending on your budget and mechanical skills. From lowest cost and easiest to install to highest cost and most complexity. Of course the order of desirability also goes in the same order.
1) Install a transfer case lowering kit which consists of six spacers that are inserted between the skid plate under your transfer case and the frame, the six mounting bolts pass through them. Looks like this, and they are not expensive. This works by reducing the angle of the rear driveshaft which reduces the excessive angle the driveshaft's u-joints have to work into which stops them from vibrating. This also of course causes a 1" loss of ground clearance beneath the skidplate... if that's important to you as it is to most offroaders.
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The next most desirable fix is a pair of motor mount lifts. They lift the front of the engine 1" which tips the engine/transmission/transfer case angle a bit to lower the rear transfer case output shaft the rear driveshaft connects to. It works to reduces the angles much like a transfer case drop but without the resulting loss of ground clearance. This also gives the engine's oil pan 1" more ground clearance, a very good thing. You can buy motor mount spacers from
https://savvyoffroad.com/product/sav-mmltj/ or if your motor mount rubber bushings are getting old you can get a pair of complete motor mounts with the 1" lift benefit from
https://mountainoffroad.com/collect...f-motor-mount-kit-for-1-lift-97-06-yj-tj-lj-1
Best of all but also most expensive is to install a SYE (slip yoke eliminator) kit into the transfer case and replace the rear factory driveshaft with an aftermarket double-cardan driveshaft. It also requires new adjustable length control arms in the rear because the rear axle's pinion angle has to be raised up significantly to match the requirements of the new rear DC driveshaft. This is what those of us with 4" or taller suspension lifts run. Fortunately, cost and labor-wise, this is not really needed for just a 2.5" lift but if you ever get to 4" of suspension lift then it would be required.
Personally, I'd go with motor mount lift blocks if your existing motor mounts are in good condition. This is a good solid fix that doesn't lose ground clearance. Good luck with it all!