You may wish to talk with the investigating officers from the state patrol to see what they have learned about the incident and whether criminal charges will be filed. Driving with an unsecured load is a crime in Washington. If there will be criminal charges filed, you may be able to be reimbursed for your repair costs through that system by recovery of "restitution;" this would be arranged by the prosecutor, with input from you.
If this was not an unsecured load situation, and it was actually a piece of the truck that came off, you would need to look at what the driver knew or should have known about the safety of the vehicle prior to driving it on the roadway. If you can show that the driver knew or should have known of the problem with the vehicle, you are in a reasonably good position to recover the entire amount required to repair your vehicle and small claims court would be a relatively quick and inexpensive way to obtain your day in court.
Before you go that route, however, you may wish to approach the insurance company again, advising it that you are giving it one last opportunity to avoid subjecting its insured to a lawsuit and see if you are able to reach a more favorable agreement with the insurer -- if the amount is right, obtaining a check from an insurer is a more certain and simple route than obtaining a judgment in small claims court that may or may not be collectible in full.
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do you have collision coverage on your vehicle? if so the easiest route may be for you to put it through your own insurance and let them go after the trucks insurance carrier for reimbursement. many of these insurance companies are parties to inter company arbitration agreements adn do this all the time
Disclaimer: Mr. Milligram is an attorney licensed to practice law in the States of New York and New Jersey. This response is not legal advice and does not create an attorney/client relationship; rather this response is intended to be legal education only and is intended to provide general information about the matter in question. Please note that this answer does not constitute legal advice. It is merely intended to provide educational material and general information to aid the poster in finding answers to the problem posed. This answer does not create an attorney-client relationship. In most cases, it is best to contact an attorney directly to find answers to your problems.