What could be causing this bucking/stuttering in our 99 Chevy Lumina 3.1?

You've replaced pretty much everything you should to try and fix it so i can only ume at this point that you have a dead cylinder. Have a mechanic check the compressions on all your cylinders. With a dead cylinder you can either get a replacement engine or have the engine rebuilt. Shop around and you should be able to find a shop that'll rebuild it for under a grand.