linking joints in URDF and/or Gazebo?

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There are two releated Ackermann questions already which are what you want. I believe @martin_guenther s answer is the more direct answer to your question.

simulating ackermann in gazebo and planning with ackermann

I have written a Gazebo controller plug-in that implements Ackermann steering. I am going to distribute it in a package within the next few weeks. Pros: Simplifies URDF file, supports wheels that move vertically (because of shock absorbers) without a complex linkage. Con: Forces on one wheel are not transferred to the other, i.e. the wheels are driven by separate servos. I am going to try John's approach (four-bar linkage that incorporates a Gazebo joint), but have not yet done so.