I found this thread online because I was having a similar issue. I ended up getting on the phone with Skyjack and told them the issue I was having (unit would steer fine on steer only function). If trying to steer while driving, the front wheels would only turn about 3 degrees.

Plug your test gauge into the test port on manifold. Drive the machine and observe the pressure. On a firm level surface you should have 500psi. If its considerably lower, you have a bad counterbalance valve, which regulates your pressure. If it stays the same, activate steer while driving and observe the pressure guage. If it drops considerably while steering, your steer cylinder is bypassing; If pressure stays the same, your front steer spindles are bound and drive pressure (500psi) is not enough to steer them as opposed to standby pressure (2400psi) <- this was my issue.

Hope this helps.