[Cool Hand Lurker]

My front wheels shook at 20 mph too. I jacked up each front wheel and like John, my wheels settled at one heavy spot. I had to put 5.0 oz wheel weights on one wheel and 3 oz on the other one and that got rid of the severe wobble. There is still a come & go vibration at just under 50 mph so it is not yet perfect, but wheel balance will certainly create the death wobble.
It is easy to check by jacking each wheel just off the ground and see if it keeps settling at the same point on the wheel after you spin it.

By the way, I first put small magnets on the wheels as weights until I found the balance point and then weighed the magnets on a postal scale to come up with the snap-on wheel weight that I needed.