This wasn't actually during commuting (still trying to find a good time to do my first partial bike commute

), but it was during practicing with my new mirror on the MUP, so I'll stick it here.
Actually, it's happened on all of my rides, but I first connected the dots now.
Basically, if it's anything other than a short ride (<2 mi or so), with downhills... if I get going quickly on those downhills, my legs start shaking horribly when I coast (which is when I'd like to stop to keep the lactate burn from getting bad.)
At first, I thought it was something wrong with my bike, because it seemed to go away as I slowed down. The tires were dryrotted, after all.
Replaced the tires, and did a ride today. The shaking was still there. One time, I came to a stop, and was still balanced on my bike... and the shaking was still there when I came to a complete stop, which tells me it wasn't the bike, it's ME. Also, after the ride, my legs felt like jello. (And it wasn't even a long ride...)
It seems to be better if I coast with one leg all the way down and one all the way up, but that (other than the lack of shaking) seems to be less comfortable.
Ideas?