You do not have to have a strong defense. What those offenses have are shit run games and extremely potent passing games. Their defenses, while crappy vs the numbers generate a lot of turnovers. And that is the stat that matters the most defensively.
And as for your 18th ranked defense from last year, if you are using total defense then that is not a true indicator of how good you are defensively. For christ sake the Chargers were the number 1 ranked defense last year. This stat takes into account yards per game allowed. The chargers didn't allow that much because they were giving up special teams TD's like crazy. Against the pass you ranked 11th which was pretty good but you were in the bottom 3rd against the pass which needed to improve.Last year you played the NFC west in 4 games (arguably the worst division in NFL history), and 4 games against the NFC North, which had 3 of the bottom 12 offenses in the NFL.
You guys lost Richard Marshall this offseason and he has proven to be a decent CB.