I'm new to the hobby and need some help with a Traxxas Rustler with a Traxxas 2018 steering servo that is slow to respond and chattering. If the front wheels are picked up from the ground, they seem to turn, but when they are on the ground, they don't travel all the way either left or right. Does this indicate a bad servo or servo saver or some other problem? Any suggestions?

The Traxxas 2018 is a standard torque, plastic geared, no frills servo. If you have had a crash or two you might have stripped a few teeth off one of the gears.
I suggest a "high torque, metal geared" servo. This will end your problems permantly, as well as give you more positive control over your steering.
You might try a Hitec HS605mg. These are a decent servo for the money.
There are many different brands and prices of this style servo. Any one of them would be a fine choice. It's all up to how much you want to spend.

Thanks for the reply, we have had a couple of crashes, so that may be the trouble. I thought the servo saver was supposed to prevent stripping the gears, though. Regardless, I'm going to upgrade the servo when I replace it.

what upgrade servo did you end up going with?