What it's called escapes me currently but it's exactly what you described, a vibration dampener. Often times automakers place these rubber blocks along the underside of the car to help absorb the vibration and noise generated from both the road and the vehicle itself
It's called a DAMPER, TRANSFER DYNAMIC #33183-35050. It's a $350 part so i'd recommend getting one from a junkyard if you can.
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1999 Limited with rear locker.
Sherpa Matterhorn | Sonoran Steel Diff Drop | SPC UCAs | DuroBumps Front and Rear | OME 890 Springs and Black Tokico Shocks | Eimkeith Reinforcements and PCK | King Coilovers | OCD Sway Bar Links | Opt Offroad Rear Arms | Lil Skips Gas Tank Skid Plate | FGP Rock Sliders | 4x Innovations Rear Plate Bumper with tire carrier