Only feel it when brakes applied hard above 45mph or so.
Replaced back brakes including drums and hardware- somewhat better-
Front rotors are fairly new, checked them, aren't warped.

Brake pedal does not pulse, doesn't pull-

Any suggestions?

Welcome to AF. You checked the ft rotor runout with a dial indicator? Apply the parking brake a low speeds see if you feel the same shudder.

If you have the alloy wheels, they MUST be evenly torqued at 100 ft/lbs or they will distort the rotors and cause this....if you don't do this, you'll be chasing your tail trying to find it...

Thanks to you both- I didn't use a dial indicator, just a straight edge- but that probably is the next step-
Also had a thought about the valve that distributes the pressure front/rear, can't remember the name of it now.
Has either of you had a problem with that?

No alloys on this mighty Lumina...

The proportioning valve is not your problem here.

I agree with you at this point, tried the parking brake - no shudder, so the back ones are good-
Will have to use dila indicator on the disc s in front- can't be huge as I rotated and eyeballed with a straight edge-
Guess I'll find out how well auto zone stands behind their stuff- they are only a few months old!

Anyone have bad experience with autozone?

Thanks for the help!

Anyone have bad experience with autozone?

Has anyone NOT?

Another thing to check is your outer tie rod ends. If worn out, (fairly common issue on GM W-bodies), they can cause a shudder under hard braking above 40'ish that won't show up under light braking. Normally, warped rotors will cause a pulsation regardless of speed or pressure applied, it will just be less noticeable.