I've just purchased the Liberty (2004) and I'm not sure how it's supposed to behave on the road. It drives wonderfully most of the time, however, if I take it out on a motorway, going anything over 70mph the whole car starts vibrating. It doesn't seem to be a wheel balance issue, as I've experienced this before. Unbalanced wheels give more of a steering wheel judder, this feels and sounds like the entire vehicle is vibrating.

Check the U-joints in the rear driveshaft. Sealed-for-life units are recommended rather than the greasable type if replacement is needed.

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2005 KJ CRD Limited 4x4:
245/75R16 BFG TA KO2s
OME / Clevis 2.5" Lift
JBA Lifted A-Arms
IRO WJ Short Rear UCA/WWDiesel mount
Skid Row Skidplates
HDS Model 001 Thermostat (190°F)
Suncoast TC
Full Weeks Kit
Bosch 5V glow plugs
Hayden 2986 fan clutch / GM 11-blade fan
Samco / Sasquatch Intake Hoses
Carter in-tank pump
Provent 200
V6 Airbox

Take it to a dealership and have them look it over. Could be a u-joint but there is a recall on lower control arms that they should fix for free if it hasn't been done yet. That could make the vibration and if it fails would be crazy dangerous!

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GDE FT Ecotune, EHM, Weeks kit stage 1 & 2, Carter 4600 lift pump, Upgraded tranny, Fishing boat hauler!
New at 164K: head gasket, rockers, exhaust valves, ARP head studs, Injectors, 5v Bosch glow plugs, water pump, timing everything, serpentine everything,
New at 225k: new head, timing belt

both are very good pieces of advice but most likely universal joints assuming there is a lot of mud in wheels etc.( like the movie My Cousin Vinny)
For safety of life and limb make sure the lower control arm recall has been performed if there is one on your model.

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05 Blu LIM, OME GDE Tbo, wk II 245/75/17, KC Lights, bull bar, 195K H TC
05 Blu Lim, Dayton, GDE HT, 255/75/18 , 210K , H TC
06 Bla Lim, GDE Eco, Destination AT 245/75/17, 151K, H TC
06 D Khaki Lim 126K wkII Eur TC
05 D khaki Lim 145k refurbishing
All CRDS: Fumoto, Lift , Fan Shroud mod, fuel head Gen II, SAMCOS,self TB, 2 Mic filt, Hayden