The first time I towed my Jeep over the mountain pass this happened to me. Very scary. In addition to passenger rated tires check the tongue weight. Too light of tongue weight can be bad. I also had maintenance issues at the time with new to me truck. Before the next trip I took the setup to local weigh station after they were closed and moved things around to figure out the proper tongue weight. Definitely get rid of the car tires. Ask your local trusted tire dealer what they recommend. Probably going to be D rated LT tires.