If your receiver hits the ground on a speed bump then something's wrong. The receiver should be positioned just under the bumper, not touching it, but maybe a finger's width below.
If it's just when you have the drawbar in place, then maybe you have the wrong drawbar; it should have about a 4 to 6 inch rise, raising the hitch ball up to a height about even with the middle of your bumper.
This is based on my experience with my last minivan (Pontiac). We just got the new Sienna couple days ago & haven't added the hitch yet, but with the Pontiac, the receiver hit the ground only on transition to a very steep slope (our driveway), and I expect the same with the Sienna since it has about the same clearance.
We deal with it by approaching the driveway at a very shallow angle, and never with the drawbar in place, but that's beside the point.
BTW, how much did your dealer charge for installation? Mine quoted over $500, but the truck accessory shop that did my other car quoted $160 with wiring & everything!
good luck,
jtw