rockcrawler

I have a 2000 XJ and my ABS light is on, when i first crank it up it stays off until i get up the road and hit the brakes, then it feels like a brick behind the pedal and the pedal starts vibrating really bad, after the first couple of times I hit the brakes it goes away, then its fine until you let it sit for a little while? I Just changed my front calipers, rotars, and pads, drove it for a little while and it was fine, then i broke a U joint, tore everything back down fixed it and now my brakes are like this... help !!!