It's gotta be the calves! (and the shoes, to keep onto the commercial line)
I'm telling you, whenever I use basketball shoes, even Kobes which aren't really that heavy or bad quality, I feel like my feet are into cement. I can't move around well, my GCT in moving around, not to mention jumping, is soooooo long it's ridiculous, yet when I take running shoes on, which are light and have a foam sole, all these problems seem to go away. I feel like despite me increasing in bodyweight over the years - I haven't increased in calf strength pretty much at all - I bet I'm at around the same ratio of calf strength to bodyweight or so like I was when I was 40 lbs lighter...
Except those 40 lbs x 10 times = 400 lbs more in a high speed plant so... after "some" point it becomes such a great overload for the calves when you get heavier, and I feel like I'm at that point right now.
When I do single leg bounds I have more of a tendency to collapse at the knee/hamstring, and remember I can't do alternate leg bounds due to my right knee so I'm limited at LLLL ... that's why I'd try backward depth drops...
Another thing that passed through my mind is walking backwards and downwards on a hill slope. It would probably be even better for the calves, that continuous tension in them... sure, dangerous and kinda silly, but still. I think it's worth the try and see how does it feel like. I'm not sure I've ever seen anybody do it yet.