So, I tried out the back extensions today. It was great for the lower back, but I didn't feel it much in the glutes/hamstrings. I had the pad low enough, right around the crotch.
I was also getting this dull pain on the medial side of both of my knees. This has been an issue for a few weeks, and I can't really figure out what's causing it. It seems to be whenever my legs are locked out.
My best guess is that lying leg curls and calf raises on the seated calf raise machine (it mimics a standing calf raise, your knees are straight) are both causing issues at lock out.
I haven't really been able to appreciably mimic the pain during any exercise, except today during back extensions. I've done back extensions before, as recent as a month or two ago, with no issues. But I used to have the pad set much higher, at the point of my waist.