i ain't no expert but might be the head gasket.
4*55 = 220 miles
55mph
55mph
+/-55mph mph
52-55mph
55mph
55mph
45-55mph
The problem is that you have engaged overdrive to early or the transmission govenor springs have weakened and is engageing it to early. if you shift indacator has 2 d's on it, put it in the one without the circle around it. if it has 123D then have it in 3 if your not goin over 50 for most of the time. only engage overdrive if your going to be driving over 50 for most of the time. when it begins to shake try steping down further on the gas to make it downshift out of overdrive and you will feel the shake go away. if none of this works then have you wheels rebalanced and your suspension and wheel bearings checked. but try the shift solutions first. Sincerely Earache :>)
Loose suspention is usually the cause of this but it also may be a loose wheel have you had any work done in the last fifty to onehundred miles?
It could be a bad motor mount or mounts. Motor mounts have rubber in them and go between the engine and the frame. Over time the rubber gets brittle and wears out, causing the engine to shake.
Get the tyres checked for balance, and bulges in the side wall.