I don't think your issue is due to not having an underpan. Back in October, I drove for two weeks without one and did not have any issues, even going at 80 mph. Just this past Tuesday during the blizzard, I drove over a snow covered median while taking a left turn. The next morning while driving on the expressway, I had a horrible shaking in the steering wheel going 65+ mph. I took it in to Firestone and purchased their lifetime alignment. They told me my alignment was off and there was a lot of snow packed into the wheels and around the front of the car, which could've lead to the shaking. Luckily, I did not bend any wheels.The next day, the car drive just fine after getting the alignment and clearing the snow from the wheels. For you, it could be the same issue.