Vehicle shakes at speeds over 60-65, Too much play in the wheels. on 1991 Ford E-150

Our van conversion usually shakes or vibrates at speeds over 60. Sometimes it isn't as noticeable but maybe we get used to it. Also there seems to be too much play in the steering wheel when steering left or right. In other words, the wheels do not respond as well as they should when steering the van. This is scary when you're driving in place like Crater Lake or some other skinny mountain road. I'm wondering if the two symptoms are part of the same problem.

The person we bought this from is a mechanic and assured us the van was in good shape. The front tires are a not a wide as the back tires - all tires are "custom" sizes. The tire man said it shouldn't make that much difference but logic tells me that it would. Maybe I'm just hoping it's as simple as wheel size differences.

Asked by for the 1991 Ford E-150
If your tires are different sizes that will be a problem. All your tires don't need to be the same make but the same size on the front and same size on the back. Ckeck the steering box on the top of the box there is a adjusting nut with a plain screwdriver screw. loosen the nut and adjust the screw in or out until the play in steering wheel is gone. If that don't work get tires reblanced.
Can you tell us where is the steering box that you had mentioned?