I have a 2018 Silverado 1500 LTZ 6.2 L gas. 150,000 miles on the truck.
8L90 transmission (POS).
My 4wd Service Light just came on.
It will not allow me to enter any of the 4wd modes at all.

I do use the truck for the beach, but I have not been on the beach since
May of 2025 and no need to be in 4wd since then.

Is there any way i can troubleshoot this myself. what is recommended?
a. Personal Mechanic
b. Chevrolet Service center?

Any help will be greatly appreciated.,
Happy Holidays

Lou