Vibration

When driving at 2200/2300 rpm and then slightly aceleratine to 2600/2700 rpm the van shakes real bad (not the steering wheel) It feels like the whole van, and seems to not have much power. Then if I increase the rpm to above 3000 everything is back to normal

Please supply the missing details, including:

Model year
Engine type
Odometer mileage
Whether the vehicle is still covered by warranty
The vehicle’s maintenance history in detail. Please don’t give us an essentially meaningless statement such as, “it is well-maintained”.

Disable overdrive and test drive it.