2003 Ford Transit van shakes whilst accelerating

When I accelerate in 3-4-5 upto 50 mph the whole van starts to shake as soon as I take my foot off it stops I have checked wheel bearings also had all the wheels re balanced there does not appear to be any knocking noises I have been under the van as far as I can see the engine mounts look ok but I could have missed something this is on a transit connect.

Shakes on acceleration, stops when not on gas. Change one wheel with a dent, replaced 4 tires and all balanced and aligned and still shakes on acceleration