Jared posted the offensive playoff rankings yesterday, including multiple playoffs odds tables and some general thoughts on targets.
The same principles apply here as well, and I used the same basic approach in ranking the defensive players. But there's room to alter your strategy here and/or to build off what you've already selected on the offensive side.
First off, you probably shouldn't be chasing IDPs too early in your playoff draft -- or otherwise targeting defenders at the expense of highly rated offensive guys. The top offensive players are most likely driving the scoring in your format. Beyond that, the top seed in each conference drags along weak fantasy LB groups. If you want Chiefs pieces, I'd target the DBs over anyone in that LB corps.
Whether it's the Chiefs or another specific team or 2, feel free to skip down the rankings to the next name from that team when you're looking to select an IDP.
Or you could use defense as an opportunity to hedge. None of us will be correct in all of our playoff picks. For example, let's say you're primarily focused on the Chiefs but realize that the Bills could knock them off. Consider Bills LB Matt Milano, who returned to full playing time (88%) in the Week 16 win over the Patriots and even played half of the Week 17 win over Dolphins.
Beyond specific situations like that, there's always the question of whether to pick a good player from an underdog (say, Roquan Smith) or a marginal option from a favorite (such as a Chiefs CB).
With all that in mind, here's a starting point for planning your playoff IDP roster. Feel free to email with any further questions.