shakes when maintaining speed

[Quacked Up]

I've seen a couple of posts about the issue but can't seem to find them now. I have an 03 Explorer 4.0 liter XLT with 130K. It gets the shakes at 40, 50 or 60 mph when trying to maintain that speed. Accerelate and it goes away, let off the pedal and it goes away.

Changed spark plugs and MAF sensor per recommendations but still gets shaky.

any other suggestions?