For me it was the driveshaft, very noticeable at 45-65miles under load with acceleration, vehicle would sit at 2750rpms keeping speed going up large hills at 60mi, kick it down a gear made it seem better, 32.5" tires 4.30 ratio. Bought a new aftermarket DS with spicer joints, seemed better for awhile, 1000miles, vibe and a whom whom you can feel in the tcase shifter came back worse, but really was never solved at highway speed, replaced rear diff ECGS, then pressed my own OEM bearings on, same vibe, PCK welded on, new poly rear body mounts, all new front and rear sway bar bushings and links, complete new oem/ome/bilstein suspension, adjusted the suspension heights and all static OEM bushings, front LCA whiteline bushings, OEM OTR and LBJ, dealer aligned x2, found play at cardan joint, needle greased the aftermarket centre bearing made it better for 20miles, added sound deading, replaced transmission mount, OE plug wires, coil swap from good rig,.. bought an OEM DS, vibe disappeared, but a noise persisted. Exhaust drone with 2.5" cat back, no CEL codes, replaced original TPS with OEM, because I wasn't seeing 11-25% TPS on scan gauge and nothing in-between and hard off/on throttle response and shifts, new TPS shifts and drives better, less exhaust drone as it doesn't sit at 2750rpm under light throttle going uphill, smoother vehicle, much more consistent TPS and LTFT readings. Tires and rims swapped on and off completely, main culprit was DS and TPS ironically, everyone will have a different experience I realize. I should have checked the rear axle shafts for runout, but seems smooth now so never bothered, but that could be a hard one to pin point and wise to do while you're at it.
