Hmm . . . that would make me think your tires are not to blame, unless they’re ancient and hard as a rock
The steering wheel is not shaking when this problem occurs, is it?
It could be driveline induced, perhaps related to the driveshaft. The u-joints may be bad, or perhaps the driveshaft itself is imbalanced. A dent could cause this, as well as a balancing weight which has been lost
This vibration you feel . . . may I assume it’s not present at low speed, such as 15mph?
And the rumble, you can feel it in the seat of your pants, more than you can hear it?
It’s more something you can feel, versus something you can hear?
Are you pretty handy?
Good assortment of hand tools?
Anywhere you can park the van for a few days?
As for the brakes, if you hit the brakes at freeway speeds . . . let’s say 65-75mph . . . does the brake pedal kick back violently? And does the whole van seem to shake when you do this, and you can feel it in the seat of your pants?
If it’s only the pedal kicking and no shaking, it might only be the front rotors warped
If it’s a combination of both, the problem is more likely the rear drums, as that’s where the parking brake shoes are, and your friend probably caused the drums to go out-of-round
You can feel the vibration in the seat of your pants at 45-55mph, regardless of if you’re stepping on the brakes or not?