6 week old australorp having trouble walking - not sure what to do

[GilmanChick]

Hello all. I'm a new chicken owner. I have searched the internet for hours learning how to care for our chickens, but I can't find an answer to the problem one of my chicks is having. About a week ago (she was 5 wks) we noticed that she looked as if she were shivering and squatting, kinda like they all did when working out a poop, but there was no poop. I gave her a little yogurt and sweet potatoes because I thought she must be constipated. But she is pooping and it looks normal. After watching her more closely over the last week, and now that she's getting a little bigger, it seems more like she has a problem with her legs. She runs, but then stumbles, sometimes falling on her face. When she's standing, her legs are quivering and she seems like she just can't balance herself. She spends a lot of time sitting down, still pecking and moving around with her 5 siblings, but sitting whenever she stops. We've looked and felt her legs, granted we have no idea what we're looking for, but there's no obvious injury. They lived in a cardboard box with fine sawdust in the bottom for the first 5 weeks. We did put them in a crate with a wire bottom that grass could come through so they could have some outdoor time. We did that for a couple of hours almost every day after they were about 3 weeks old and the air temp was the same as under their lamp. (It didn't seem right to leave them in a cardboard box all the time.) Don't know if this could have anything to do with it, but we wormed them at 4.5 weeks with Wazine 17 because we saw worms in our adult chickens' poop. The babies and adults haven't been together yet, but they have visited the same areas of the yard, so we put the med in the babies' water the same time we did the adults'.
It's hard to see her struggling to keep up with the others. I just don't know what to do to help her. Thanks in advance for any suggestions you may have.