try changing the spark plug wires also
I had the same problem: at 50-55Mph, when accelerating, the engine would start shaking. When I released the acceleration the vibration went away. Also, it would shake badly when driving up hill and accelerating. Went to a mechanic and changed both catalytic converters (the first one melted and clogged the second one) did a complete tune-up, wheel balance and alignment. He also changed the rear engine mount. However, the van was still shaking but it was 50% better already. Next, he changed ALL engine mounts (supports) AND the right axle (drive shaft). Now the van runs like a champ and I could not be happier! I hope this helps. If you can do your own replacements you can save a lot of money!
A little more information please What model year of Ford Expedition and what is the size of the engine ( 4.6 or 5.4 ) The first model year of the full size Ford Expedition was 1997 , so things have changed
The 2006 Ford Expedition has a V8 engine.
The 2013 Ford Expedition has a V8 engine.
The 2010 Ford Expedition has a V8 engine.
The 2011 Ford Expedition has a V8 engine.
The 2008 Ford Expedition has a V8 engine.
The 2004 Ford Expedition has a V8 engine.
The 2014 Ford Expedition has a V8 engine.
The 2003 Ford Expedition has a V8 engine.
The 2009 Ford Expedition has a V8 engine.
The 2012 Ford Expedition has a V8 engine.