These are what I mean by the steel balls. Actually they're the little hollow pivot balls in the suspension (tie your shocks, rockers, and other suspension bits together). Stockers are plastic. http://www.thetoyz.com/cart/thetoyz_...%201/16%20REVO
I've just read they're a good upgrade over at the traxxas forums; I don't have them myself.
The stock shocks seem to have a steel hollow ball at the "inboard" mounting locations (the upper ends of the shocks, closer to the motor), but plastic hollow balls at the "outboard" mounting locations on the the rockers. I don't know if upgrade shocks come with metal ones....i guess it would depend on the brand maybe.
Haha, I ordered 4 of those hobbycity zippy's due to fear of DOA batteries (I only wanted two). All four seem to work fine for me, and now I have a spare set. But it's not a big deal as they're insanely cheap.