It actually wasn't Heather (the deaf girl) but a different girl from before she came along . . . first or early second season I believe. I think the girl's on Popular now, though her name escapes my memory. She's really cute though.
Bellytickler, I didn't mean to imply you were lying, I was simply stunned that I had missed that scene and was hoping someone could provide some more details about the episode so I could be ready when it airs again (it airs every weekday in syndication in my market).
I've seen several very quick instances of what appear to be tickles between Mary and Lucy on the show, but they're so brief and indistinct (and likely spontaneous) that it's hard to tell for sure. I'd love to see a more drawn out scene like the one you're describing - those two cuties are high on my list.