mine just slipped into groves, so it pulled right out. I found a metal yard and got stainless steel bars to replace them. They were a bit loose, so I put fabric down and then pushed them into the slot. They held fine, though I sold not long after that.
I never experienced a wobbly carriage, but would suggest making sure the rails are tightened correctly, and everything is very level.
BTW - here is an old picture of what I had.
