Van that started to shake after a car wash only at high speeds. The vehicle was ran on a muddy gravel road before the car wash but there was no shaking at high speeds after the gravel road?

It could be as simple as a wheel balance problem. Jack up each wheel and examine the inside for mud and debris. There could have been evenly distributed mud before the car wash but some of it could have been washed away leaving the wheel unbalanced.

It's also possible that you knocked off a wheel weight and now the tires are out of balance.