Abs
Have the axles been swapped? If so, no ABS. ABS can be trouble on the trail, but it can save your **S on the street. The axles that have ABS will have some sort of sensor attached.
Plug it in. To test - go to a level dirt road/fire service road. Get to 10 to 20 MPH and slam on the brakes. Hang on and get ready. The brake pedel should pulse and there should be no lockup. The truck should stop straight. If you feel the truck begin to yaw, get off the brakes and get it under control. Don't risk a roll.
You will probably find the PO hacked something and the ABS is dead. You know, bad pump, or something.