I know this company makes watts links for a fox body. I don't know much else about them.
http://www.evolutionmsport.com/
I know the typical arguments for one over the other involves the greater weight off the watts over the panhard. One advantage to a frame mounted watts link is that the rear roll center height is fixed relative to the chassis. With a panhard the roll center height changes with respect to suspension travel.
Oh and the watts link won't create any side movement in the rear axle with suspension movement - the panhard will.
I'm sure some of the suspension experts can discuss this in more detail.
With a Fox platform, I think either will help the "snap oversteer" created by the bind in the stock rear suspension. Adding either device to the stock suspension is redundant, but works. Probably the best solution is to either go to a 3-link w/ either panhard or watts, or a torque-arm with panhard or watts.
Later,
Brandon