The fact you stating the RPM's don't go higher even though you're pushing the gas is really strange. I don't think that would be related to the spark plugs, although hopefully you used NGK's or DENSO's.

I'd personally install a brand new MAF sensor in there, and not purchased from Ebay like I did. Buy from RockA... or another reputable seller, NOT Ebay. It's probably the original MAF in your LS?. Throw the LS a bone and give her a new MAF for about $65, and takes 3 minutes and a phillips screwdriver DIY. If you're a worrywort, disconnect the battery first then replace the MAF.

While the MAF or plugs probably aren't related to your issue, sounds like a restriction related to you not able to go over 3500 RPMs on the highway!? Perhaps you idle or drive it for 10+ minutes then shutoff the vehicle and take a look at the cats on each side of the vehicle? Make sure neither are glowing orange or anything crazy like that and they are of equal temperature? (will be hot) Or you could even take notice the color of the metal when the vehicle is cold? They should be similar looking in color, not one silver and the other white. Even better would be a laser temperature sensor if you have one and check the cat temps after idle for 10mins?

The rattle could possibly be the internals/honeycomb/screen/catalyst inside the cat, after they've broken apart. We don't hear many cases of bad/plugged/damaged cats on the forum though. Once in a while yes, but the last CL member that I recall had his exhaust pipe plugged up with snow, and that is what damaged his cat. Is there anything you recall that could have plugged up your exhaust pipes? I see you're from NJ (dirty jerzee!) but we didn't have any snow this year.

 

Check the gas pedal the car is not cable driven it is all electronic from the accelerator to the throttle body if the pedal was bad it would exhibit the exact symptoms and not throw a code.

 

I just got the cat replaced after the y pipe and that was the issue. I also bought a new MAF as well. Thanks for the help. Car drives great now.