Jack both front wheels up into the air and sit on your but looking at one of the tires and grab the front of the tire with one hand and grab the back of the tire with the other. Shake it from left to right. Can you say sloppy and I need some new front end parts?

I run a F-Body Road Runner at similar MPH. If you find no defects in the rear mountings, I would check the front end. The front suspension is quite different on an F-Body. Is the K-Frame still mounted with rubber insulators, if so they need to be swapped to metal blocks. You can fab them, used to be able to buy them as well. Second is the center link level to the ground? Many had the problem of the idler arm bracket being located incorrectly. The bracket on the K-Frame can be heated and bent into the correct location very easily. Third the torsion bars also act as the strut rod stabilizing the lower control arm. If either end of the torsion bar bushings are loose you can induce caster and toe change. Forth make sure the steering box mount isn't cracked. The rest is pretty straight forward as far as worn parts and proper alignment. Good Luck.
Doug

Many times a rear end "sashay" similar to this is actually a harmonic problem in the frequency of the suspension front to rear, often cured by stiffening the damping of the front shocks to kill or reduce the harmonic.

Is this happening only on decel? If so, what is the method of slowing the car? Does it happen without the brakes being applied?
Does the engine induce significant engine braking? Do you have a spool or a locking style diff? Does the car still have a front stabilizer bar?

look at the bump stops on the front end. My barracda would try hanging a hard right as soon as I lifted because one side was diving down.the bump stop was gone on one side

I was watching the 8 second Hemi Cuda's at the finish line at Norwalk a few years ago and some of them were scary to watch as they would rock/roll side to side at speed and even more when they got on the brakes. And one of the Dart's looked like it was gonna roll over on it's roof when he slowly turned into his pit spot. Not sure why.

Thanks for all the replies guys. The K member is solid mounted to body. The front wheels have no "slop" in them due to all new parts at the wheels, no front or rear sway bars. Steering box it tight to K member. It starts as soon as you lift off the gas and gets worse as the brakes are applied. I got it out of the trailer and inside in our warehouse on flat concrete. The diag link for the ladder bars have slop in it. The brackets ARE wore out. I am going to pull them and replace with longer bolts, recheck the rear susp then get out on the 4 lane hwy and make a little hit with it to see. I will post findings.
Thanks everyone again.. You guys are the best!

Sweet F-Body.