van starts to vibrate or shutter starting at around 50mph gets worse faster u go

What is your question?

THIS PROBLEM SEEMS TO HOLD THE VAN BACK FROM EXCELERATING. IT IS NOT THE TIRES, AND ONLY SEEMS TO DO IT ON WARM DAYS 90 DEGREES

What makes this problem better or worse?

ok at slow speeds/starts at around 50-55 mph

How long has your 2008 Dodge Grand Caravan had this problem?

3 days intermittent