2000 Jeep Grand Cherokee rattles when braking

Sounds typical of a warped brake rotor. Have them checked for thickness variation and lateral run out. Also, if they were off recently and the problem started after they were reinstalled, look for debris stuck between the rotor and hub, especially the round spots of rust that form on the backside of the rotor by the access holes in the hub.

The easiest thing to search for is the noise, then hope it's related to the cause of the vibration. There is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.

Since you already replaced something in the suspension system, if any of those parts are mounted with rubber bushings, and those parts rotate, like control arm bushings, be aware they have to be tightened while the vehicle is sitting at normal ride height. If you tighten them while it's suspended on jack stands with the suspension hanging down, when you lower the vehicle, those bushings will be clamped in a permanent twist. That will reduce their life and can tear them in short order leading to a recurrence of an elusive noise or vibration.