i've had this same thing for 75k miles with no change in ti
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sounds like that^ no rpm change no felling in the seat, i just figured it was harmonics of the drivetrain. it makes me sleepy
your's probably is the mount. there was a TSB on the transmission mount causing a harmonic vibration at 1500rpm at highway cruising speed.
'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.