Standard Disclaimer: I’m not licensed to practice in your jurisdiction, and I’m not offering you legal advice. I’m not your lawyer, you’re not my client. If you need legal advice, contact someone licensed in your jurisdiction.
Now, with that said, as to the general theoretical question “can I be legally liable if I lend my truck to someone and he gets in an accident.” The answer is yes. First lets deal with the liability issues, then we’ll come back to the question whether your insurance company would cover any resulting losses.
Previous posters have already covered one basis for liability: if you are negligent, i.e. don’t use the amount of care a reasonable person would use, in maintaining the truck. Extreme example: you took all the lugnuts off, forgot to put them back on, and then lent the truck to someone without telling him. As he’s driving, the wheels fall off, the truck crashes and somebody gets hurt.
But even if your truck is in tip top shape, you could still be negligently liable for an accident if you were negligent in your decision to permit a really bad/crazy/drunk driver to borrow your truck. Extreme example: your friend Crazy Larry, who you know has been in 12 auto accidents, shows up at your door at 3 in the morning, clearly drunk and says “hey can I borrow your truck, I just need to drive around for awhile so I can clear my head”. An person exercising reasonable care would NOT lend their truck out to Crazy Larry under these circumstances. Now, of course, if you did lend your truck to Crazy Larry, and he hit somebody, they could sue Crazy Larry. But let’s say Crazy Larry is penniless. Now what? Well, now the victim comes after you for negligently permitting Crazy Larry to use your truck.
These are not the only circumstances under which you could be liable, but they are the main ones I can think of.
Now, the next question is whether, in the event of a loss, your insurance company would cover you. That question is entirely separate from the question of your liability. That is a matter of the contract between you and your insurance company, and whatever regulations govern that contract.
Broadly speaking, here’s how auto insurance works. You get sued for damages arising out of an auto accident. Let’s say under one of the circumstances discussed above. Then the question is, is subject matter of the suit covered by your insurance contract. (As you can imagine, this question can itself be the subject of litigation if you think you are covered and the insurance company thinks you aren’t). If yes, the insurance company defends the suit: they pay for the lawyer, etc. And, if you lose the suit, the insurance company pays the recovery, UP TO THE LIMIT OF YOUR POLICY. Anything above that, you are on the hook for.