Since you didn't fill out the profile with Dad's problems or your location, the amount of useful replies will be limited until you post more information.
Has your father been declared incompetent? Is there a POA? Did your father or the POA decide to allow the registration and insurance to lapse? Was your father really irrational (driving regardless of registration/insurance status) or merely forgot the registration and insurance had lapsed? Is your father physically capable of driving if he could get into the driver's seat? Mentally capable? Is there an expectation he will eventually recover enough to ever drive again? Has your father's doctor said he shouldn't be driving?
Have you tried talking to your father about his driving challenges?
If your father is incompetent then I would remove the truck from his home and probably tell him it's in the shop for a repair.
If your father is competent (just forgot no registration/insurance), talking with him hasn't helped and you really feel he is a danger to himself and others on the road, I would disable the truck (disconnecting starter wires is my favorite, difficult to see unless you crawl under the truck) and offer to take it to the shop when he discovers it won't start. Contact his PCP and see if the doctor is willing to tell him he shouldn't be driving.
I am fortunate I never had to go through forced restricted driving. My mother stopped driving when I asked her about a year after her MCI diagnosis. I did promise to take her or make sure she had transportation to anywhere she wanted to go. My father with vascular dementia restricted his own driving after getting lost in town and rear ending another car. He agreed to only drive the dead end road he lived on, although he did know that I was going to get involved if he tried to keep driving on public roads. Dad still wanted to drive from his house down a straight stretch to a former gas station building about 1/10 a mile away where he had a garage. Since this was a thinly populated country road with no children, that didn't seem to be much of a risk. He owned all the property between the two, he could have legally driven without license, registration or insurance if he stayed on his property and didn't use the road.
I would go over there with a guy friend, or one of your dad's friends or relatives who will understand the issue. While you distract your dad with something have your accomplice disable his truck by removing the battery or distributor cap, or siphoning out all the gas, etc. If you don't have durable Power of Attorney or guardianship over him then if you tow the truck away he can technically call the cops on you (plus you're not the owner of the truck so not sure the tow company would come out). You can't drive it anywhere if it is unregistered. If he tries to drive and you call the cops he will get cited and his truck will be towed to a yard at that point. Seems expensive and no guarantee that he won't hit someone as he drives.
Have you taken his DL and turned it in to the DMV? Does he just have an ID now? Let him no the repercussions of driving non registered non insured vehicle. Possible lawsuits. He can lose any property or money that he may own. He can losehis lifestyle by going to jail. Sell the vehicle....
Has your father been declared incompetent? Is there a POA? Did your father or the POA decide to allow the registration and insurance to lapse? Was your father really irrational (driving regardless of registration/insurance status) or merely forgot the registration and insurance had lapsed? Is your father physically capable of driving if he could get into the driver's seat? Mentally capable? Is there an expectation he will eventually recover enough to ever drive again? Has your father's doctor said he shouldn't be driving?
Have you tried talking to your father about his driving challenges?
If your father is incompetent then I would remove the truck from his home and probably tell him it's in the shop for a repair.
If your father is competent (just forgot no registration/insurance), talking with him hasn't helped and you really feel he is a danger to himself and others on the road, I would disable the truck (disconnecting starter wires is my favorite, difficult to see unless you crawl under the truck) and offer to take it to the shop when he discovers it won't start. Contact his PCP and see if the doctor is willing to tell him he shouldn't be driving.
I am fortunate I never had to go through forced restricted driving. My mother stopped driving when I asked her about a year after her MCI diagnosis. I did promise to take her or make sure she had transportation to anywhere she wanted to go. My father with vascular dementia restricted his own driving after getting lost in town and rear ending another car. He agreed to only drive the dead end road he lived on, although he did know that I was going to get involved if he tried to keep driving on public roads. Dad still wanted to drive from his house down a straight stretch to a former gas station building about 1/10 a mile away where he had a garage. Since this was a thinly populated country road with no children, that didn't seem to be much of a risk. He owned all the property between the two, he could have legally driven without license, registration or insurance if he stayed on his property and didn't use the road.
Sell the vehicle....