It is very hard to tell you what paperwork you need without seeing what has been filed to date, and without knowing more details about your claim. You really should consider at least consulting with a lawyer. Many lawyers, myself included, offer free consultations.
This is not unusual. Even if they have insurance, if the accident was your fault, your insurance company will be responsible. Forward the letter and anything else you receive from this lawyer or from the court to your insurance company or insurance agent.
It does not matter that your insurance lapsed (although you should correct that!) as long as the at fault driver/company has money or coverage to pay your claim. Truck cases are different from ordinary motor vehicle collisions. You need to find a personal injury attorney who has experience handling these types of claims. Good luck.
You may have a defamation claim, but be advised that it can be difficult to prove damages for this sort of thing. In order to collect more than nominal damages you would have to be able to quantify how much his statements have cost your business.
You may have a claim for medical malpractice if the metal clips were left in your body following surgery, and should have been removed. You need to speak with a medical malpractice attorney in your state about this.