Suburban shakes on Highway

Hello
I have a 2003 Chevy Suburban 2500 4x4 with 8.1lt engine .411 gearing, 110k miles. I have owned this truck for a while and have always had a little shake on the highway and at varying speeds over the years. It bothers me.

I just put new Nankang tires I got cheap from Walmart on and it's shaking more than my old, worn out Yokahama. I also powder coated the rims black, but I'm not sure if that is the problem or not. I have had it balanced twice and the last time it was one of the modern high speed balance machine only some tire places have. The balancer said I needed to change my steering arms and bushings. My mechanic said no. Wondering if I just bought bad tires or the newly powder coated, old custom 20" rims could be the problem??

I like the truck and the 454 engine, but I would like it to drive better on the highway. Up to 50 mph everything is fine, at 65 the cab, passenger seat and even a drink in the cup holder start shaking. The steering is a little lose but not bad for an old truck. No excessive shake in the steering wheel. Any suggestions will be appreciated.