Worn motor mount / transmission mount bushings can cause this.
When the "rubber" (usually polyurethane) breaks down, it no longer dampens engine vibration. So at low speeds you really feel the engine vibration.
When you go faster, the frequency of the vibration is higher and not as noticeable and/or the engine might twist and put pressure on the side of the bushing that hasn't gone away and it gets dampened.
Try these on-line resources for the replacement parts: www.autopartswarehouse.com, www.partsgeek.com, www.rockauto.com, www.partstrain.com - to list a few.
For example, they (RockAuto) list the replacement engine mount from around $10.00 each, and the replacement transmission mount from around $33.00 for your vehicle.
Be sure to replace all of the engine and transmission mounts at the same time, as a set - due to mileage, age, wear considerations.
Rear engine mount (for 4WD) = ANCHOR part #3157 - or it's equivalent
Transmission mount = DEA/MARMON RIDE CONTROL part #A5418 - or it's equivalent