So apparently I have a medial meniscal tear in my left knee. Woooohoooo.
On the crap side, this is why it's been making me limp like lumpy for the past couple of months, as well as hurt like crazy and be super sensitive to particular motions which then result in 12-24 hrs of ache and swelling. Also: this will require surgery.
On the plus side, unlike my other knee when it hurt, this is a specific, visible problem. Also on the plus side, as my ortho surgeon said as we looked at X-Rays while waiting for the MRI, "There are three things that might be happening in your knee. One of them I can't fix. One of them I can fix in 20% of cases. And one of them I can definitely fix." This is the 'definitely fix' case. Hooray! Also, since it's arthroscopic surgery (no incision) the recovery is fast, complications minimal, and generally I should be able to drive and put full weight on it (except for comfort reasons) within a day or two.
Looks like I'll be working remotely for a week or so in the next couple months.