ABS light is lit what should I check

You need to have the code pulled for us to further help you. These videos will help you get the codes and fix the problem.

https://youtu.be/Pb6nXV-iamI

and

https://youtu.be/rTtAnsOlZU4

Please run down these guides and report back. visually, check the fluid, look under the truck and make sure the sensors are hooked up
Roy

Wheel sensor-Scan the computer to identify which ones-

The most common failure on this vehicle by far is the speed sensor in the top of the differential.