Unbalanced wheels: The high-pressure water from the car wash might have disturbed the wheel balance. This can cause the van to vibrate at certain speeds. It's recommended to have the wheels rebalanced to ensure smooth operation.
Misaligned suspension: Driving on muddy gravel roads can put stress on the suspension components, causing them to misalign. This misalignment can lead to shaking at high speeds. Have your van's suspension checked and realigned if necessary.
Loose lug nuts: The shaking could also be due to loose lug nuts on the wheels. When driving on rough surfaces, lug nuts can loosen over time. This can cause the wheels to wobble and create vibrations at high speeds. Check and tighten the lug nuts to ensure proper wheel attachment.
Damaged tire: A damaged tire, such as one with a bulge or a bent wheel, can cause vibrations at certain speeds. Inspect your tires for any damage and replace them if necessary.
It's important to address the shaking issue to ensure the safety of your vehicle. If the shaking persists, it's advisable to take the van to a professional mechanic for further inspection and repairs.
