Like the subject says, if it's going over 55mph and I take my foot off the go pedal a vibration starts. Feels like it is coming from the rear. Vibration goes away if you give it some fuel with the go pedal. And at higher speeds the vibration is more pronounced.

Did a quick look at the driveshaft/u-joints but nothing was obviously loose/worn.

Just wondering if folks can give an idea of what might be causing this.

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2006 KJ CRD

Check the rear upper control arm bushings and center bearing for wear.

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Supporting Vendor and Moderator of LOST
05 Jeep Liberty CRD Limited
Ironman Springs/Bilstein/Shocks
Yeti StgIV Hot Tune
Week's BatteryTray
No FCV/EGR
Samcos/ProVent
SunCoast/Transgo
Carter Intank-pmp
2mic.Sec.Fuel Filter
Flowmaster/NO CAT
V6Airbox/noVH
GM11 Bld.fan/HDClutch
IronrockArms/wwdieselMount
98 Dodge Cummins 24 Valve

How are your tires? An unbalanced tire can cause a vibration. Swap front and rear tires and see if it improves.

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05 CRD; Rebuilt top end w/ARP studs, EGR delete, SEGR, 3.7L HD cooling fan, Samco hoses, in tank pump, JBA coil overs & control arms, KK clevis fork, 1/2" cradle drop, 1/4" motor mount lift, WJ rear upper trilink w/WW mount, WJ lower control arms, 4:10 gear, front/rear air lockers, ARB bumper, Rock Lizard sliders/roof rack, Mod T-stat

sounds like ujoints or pinion bearing issues

give it a good check