I have a 2005 1500 express conversion van I just bought and at 50 mph it whines and at 55 it quits or if I let off gas it quits. Any input from anyone is possibly normal not sure. Van runs great 70 80 mph no problem.

 

So you only experience these symptoms while moving? Revving in park is ok?
If so, then it sounds transmission related.

I would scan the vehicle for trouble codes.

 

What mileage on the van ?

Another source of whining could be an alternator bearing, water pump bearing or belt tensioner pulley ?
A quick check could be to remove the belt and check end play everywhere the belt is attached to. Spin all the shaft or pulley where is possible and if you can try to listen for growling noise.