As far as I know, the bumper itself is the same for all 400s with the exception of the later diesel and the Tahoe Limited and Z71 (and the Denali and Escalade, but they have a completely different front clip , period). The diesel and Tahoe Limited and Z71 have the rectangular holes for ventilation to the engine compartment (or fog/driving lights, but the holes are the same). The only other difference is if you have a smoothie bumper or the deluxe one with the slots for the rub strips and trim to attach. And chrome or painted....
The brackets and filler panels between the bumper and grille are different between the years, depends on which grille you have.
The bumper currently on my Burb,is one I got at the wrecking yard; it was laying on the ground a few trucks over from the one I assume it came on. It has 4 holes where someone installed driving lights or something, but was straight and has the trim slots. Since this was the weekend after a Hyundai ruined my bumper, right fender and grille, and I didn't know what I was going to get from the insurance, I put it on the yard wagon and continued on my hunt. Ended up being the one the body shop used, because they supposedly couldn't find a suitable replacement for what the estimate allowed.