A vibration in the steering wheel will be caused by something out of balanced, or a damaged part. make sure that the suspension and steering isn't damaged in any way, then if there is nothing wrong there, then it is a huge possiblity that the tires are damaged and need to be replaced. now, if you have a bulge in the tire, that isn't a good thing, putting them in the back may get rid of the vibration in the wheel, but there will still be a vibration, and balancing them wont fix the problem. a bulge is a sign of a damaged tire, and will eventually blow out. also, keep in mind that when your vehicle travels at a high speed, and your struts/shocks are bad, you may get a vibration due to the tires bouncing too much back and forth. I suggest taking it in and getting the front end checked out by professionals, even the tires too. if everything looks good, then check your alignment. if all that is good, then you are left to the axles. to isolate the problem from the engine, put it in neutral at the high speed, and let the engine rpm drop. if it was the engine, speed of the vehicle wouldn't be a factor, it would be the rpm, and be able to be felt at all speeds, depending on the rpm.