Why does my engine shake around 55 mph - 2005 Jeep Wrangler

Pull out spark plugs and see if they all look the same,could be dirty fuel injectors or a blocked fuel filter

If you are sure it is the engine shaking, I agree, I would check out the engine mounts. However, I recently had a shaky steering wheel at higher rates of speed, and I fixed the problem by rotating the front tires to the back.

Could be several things , If the whole car shakes it could be wheel balance or a bulge on a tire, if only the engine shakes it could be a ignition or plug problem , or worn engine mounts.

I would check the motor mounts first.