I just bought a used truck. Sometimes when I first drive the brake pedal kinda jerks like when you are sliding on ice... Soon if not right away the ABS and Slip light come on. Ive heard people say its the abs sensor. If so How would I know which one it is and where its bad? Anybody else have this issue? The 4wd doesn't always engage properly either. sometimes if you restart the truck the 4wd comes in perfect. Any advice?