Sway Bar Rattle Over Bumps

I've got a rattle noise coming from the front of my '97 Eldorado. It sounds like it's low near the floor, and in the center of the car. It happens when I'm driving from 10-40MPH over bumps in the road.

I've replaced both upper strut mounts as per TSB 53-33-04B, "Rattle/Loose Lumber Noise when Driving Over Bumps" as well as both sway bar links, as they were shot.

I drove the car with both of the links off when I was waiting for the new links to be delivered, and the noise went away completely. When I installed the new links, the noise came back.

One link (Raybestos "Service Grade" Link) was defective as there was too much play in one of the ball joints. A replacement Raybestos "Professional Grade" link (much thicker, heavier duty, with grease fittings and larger ball joints) was sent as a replacement.

Removing the links determined that the problem is not engine mount related, so the only other thing I can think of is the sway bar bushings... Should I try to replace the other "Service Grade" link with a "Professional Grade" one to be sure it's not just another bad link?

Thanks,
Nick