1996 Grand Caravan front vibration

I have a 1996 Dodge Grand Caravan. Does anyone have an idea on what maybe causing vibrations in my front end when I go over short portions of undulating/ripples in roads at highway speeds (50-60 mph)? I've had the tires balanced and rotated, but the vibrations still happen. The vibrations stop if I release the gas pedal and the van is fine. The van has no vibrations at highway speeds until I hit the undulating/ripples patches of road and not all the time too.

Thanks

I've got a 98' Voyager 3.3L, 100,800 miles. Mine does the exact same thing. There is a shake or wobble sensation that I can feel in the floorboard & steering wheel, starting around 35mph ( this wobble is only there, when I accelerate ). So the faster I go, the wobble/shake turns into more of a vibration. It's gone completely though, when I take my foot off the accelerator and coast. So this proves that it is NOT a tire balance or wheel thing.

I recently had the tires balanced & rotated, replaced both front axels with new ones ( this includes the entire axel with new cv joints ). Neither of things helped... the wobble is still there ( only when accelerating ).

Anyone have a clue as to what is the cause? Motor mounts maybe?

Motor mounts maybe?

I wouldn't suspect bad mounts in either case. My van would vibrate after letting off the gas, leading me to change all three mounts and cure the problem. From eimt's description, it sounds like worn ball joints. From bebopxx's description, it sounds like worn upper strut mounts.