ABS Brake clunk

[Dodad]

I've got a 98' 1500 Pu Brake problem. All started with fluid getting low in the rear portion of the Master cyl. ABS Light came on as well as Brake lite. Speedo stopped working below 70 mph. I replaced the ABS sensor on top of the rear end. Cured the lights and Speedo problem. However now when I stop the is a definate clunk just before I come to a complete halt. I have changed the front and the rear brakes. No change in the clunk. I shifted in to neutral to make sure it was not the trans. and got the same clunk. Also if you slam on the brakes they lock up and you skid. Don't know too much about the ABS system. And don't know what to do next. Any help will be apreciated. Thanks