Those defenses were good, but not great, and certainly not any type of league leading "shut down" type of defense. As Dolphin fans we remember them fondly, but that is only because of how poorly we have played for the past few decades. You can't compare those defenses to the current/recent Seattle defenses.
In all of the years you mentioned we only made the playoffs half of those years. Bounced in the wild card round once, and the division round the other three years. JJ did some nice things as the coach here, couple of really nice defensive drafts, but was pretty bad drafting the other side of the ball, ESPECIALLY RB and WR. We took Yatil Green before we took Madison and JT, and took John f'ing Avery before we took Surtain and Mixon, so it wasn't all puppy dogs and rainbows back then.
Jimmy Johnson was a good coach, but my lasting memory of him is his last game when J-Ville hung sixty something points on us and our defense made Fred Taylor look like Superman.