I disagree, defensive line absolutely should be their top priority this April. A solid offensive and defensive line are the keys to winning and mask a lot of deficiencies elsewhere. SF has a great defensive line and maybe average to somewhat above average DBs, which is all you need. John Lynch intentionally build SF's D-line as their strength and look at their defense. IMO, it is foolish to chase after high end DB's that will want a ton of money; they'll never be able to play to the level of their contract, they can't the way the league favors the offense. I'm all for McD finding more blue chip DB's since you still need them, it's just that they're not worth high end picks. I believe a top defense (and offense) is built from the lines out.
As far as the Jets and Redskins, or any other team for that matter, you can spend high draft picks and/or a lot of draft capital reaching for DTs and DEs, yet have a sub par defense if you pick the wrong players. You can spend a lot of draft capital and reach for a WR1, yet still have a crappy offense. i believe the Jets have a couple of rookie D-linemen, so they may be in the same boat as Ed Oliver -- they need time to get used to NFL lines blocking them, which is a lot of difference than what they saw in college.