This is what I would do:

Drive as little as possible for the next week or so until the ash has dissipated.
Hose accumulated ash off the exterior as often as you can.
When there is no more ash blowing around, wash the truck thoroughly (including under the hood) and detail with a clay bar, then use your favorite sealant or wax on the paint.
Use a pressure washer to hose off accumulated ash from the undercarriage, axles, suspension, etc.
Your air filter will protect the engine from ingesting much ash, but it may get plugged up quickly if you're driving around while the ash is blowing. Have a couple of extra filters on hand, and change them if it looks like they are getting plugged with ash (or wash and oil often if you're using the K&N variety of air filter).
Changing the engine oil and filter is a good idea, but I would wait until there is no more ash blowing around. The oil filter will protect against contaminants that are big enough to cause damage, so don't worry so much.