ABS + brake light on simultaneously

My 2000 2.2 had the ABS and brake light coming on very sporadically while driving, then disappearing after it was restarted. Now these are coming on more frequently and staying on more than before. They sometimes go off when restarted, sometimes not, and will show up soon after I start driving if they don't come on immediately when starting. I'm guessing a bad ABS sensor, but I can't get my hands on an ABS tester. Is there any way to diagnose the problem without the tester?

Have someone pull codes. It would probably help if you had the light on when the codes were pulled (don't shut the truck off). Come back with the codes and try us again :).