This brings up an interesting point. Not a fun one, but one that might be more specific with the 'decline'. The defense definitely went first in terms of talent. If you look back at wins and losses, even against ASU in 98 the D was stout. Ahead of the times. 8 guys on the LOS prior to the snap. Didn't matter if it was 4-3 or 3-4 or whatever. There was always a blitz, run or pass, LBs rushing and DL's dropping back. Take away the run and any semblance of a short passing game. QB's limited to 3 step drops and always got hit hard no matter what.
Yes, I know, we got shut out in 98. But the D held their own against a very good team. We recovered however in 99 offensively. We did not defensively. One could argue the opposite down the road and point to Pelini's good defenses and bad offenses. However, after Frank, Callahan changed things so dramatically that it's impossible to do a comparison. Even with an entire Offensive overhaul, the D can still remain strong, talented and well-prepared. It wasn't any of those things after 99/2000.
That game against Colorado marked the decline of the D. Unable to stop the run. Unable to impose will. D Line got blocked and LB's forced out of position. Totally exposed. We haven't recovered since. I do realize the Suh defense was pretty good and our anemic offense should have done more against Texass. But the defense sure lost a ton of talent quickly. That 2001 game also marks a time when we seriously got outcoached. We were blindsided by a running attack and a passing attack that was well thought out and well rehearsed. We were not prepared at all. Which also marked the end of our preparation standards in practice in my opinion. Surely we had the talent to not get gouged like that over and over again throughout the game. We lacked leadership on all levels defensively.
I think you got your years mixed up unless I'm not reading what you wrote clearly. NU got shut out in 1996. The D wasn't great in 98 because of injuries (I believe that was the La Tech year), but was absolutely dominant in 99.