During every year of the 80s, the Cardinals were near the top of the league in range factor per 9 innings. This was not too hard considering they had players like Ozzie Smith and the whole team was tooled toward defense, but we went to the world series 3 TIMES! Not to mention, that team was insanely entertaining to watch just for the defense alone (although there were plenty of other areas that were fun to watch). The Cards lead the major leagues in defense in about half the decade, they really didn't have much competition. If range factor is a bit obscure or outdated, the Cards were also leaders in fielding percentage, not a whole lot of other ways to measure defense during that time...
In the early 90s, the defense fell off drastically. Only in 1993 was it considered to be very good statistically. The rest of the decade was mediocre at best.
The defense didn't improve a lot until 2002, when their UZR were second only to the Twins. By RF/9, they were still middle of the pack, but I think things were getting better by this point defensively. Perhaps the organizational philosophy came back around to valuing defense more, perhaps free agent signings and everything aligned so that the defense was better, but whatever it was, I think it helped. Especially with Duncan's pitching philosophy, I am pretty excited to see that defense is being valued more. With players like Colby Rasmus, Brendan Ryan, Albert Pujols, Yadier Molina, etc. the team has become much more fun to watch, and reminds me a little of the 80s defense. I hope to see the team's defense improved a lot this year, with more regular playing time given to Brendan (hope he's OK!) and Colby, and no Duncan in the outfield, and hopefully a reliable year by Freese. With our pitching I think it's key for the defense to return to form. Takes a lot of pressure off of the rest of the team's abilities.
That's all I have for now, sorry if this seems rushed. I would have liked to provide some more details about the 90s and aughts teams defense, by individual players, etc but hopefully this gives you a good idea. An improved defense can only help the team, last year was pretty unimpressive for the D.