well that brings a different light on the subject
your previous descriptions were leading me to believe it might be a timeout problem due to a circuit issue in one of those circuits i have been telling you to check because you were saying that you were loosing hydraulic function and steering both. It sounded like electrical.

But, Now if you are using the hydraulic and the steering at the same time and activate hydraulic and the steering goes then it could be something in the flow or steering priority valve.

OR.... if it is still electrical related it could be something with the battery, check the voltage on the battery when operating the truck hydraulics and see what the system voltage is dropping to.
check it at the steering motor as well and see what it is dropping to.
This could be something as simple as a battery problem.
A severe voltage drop in the battery can cause the symptoms your describing. The hydraulic motor pulls alot more amps and power than the drive normally does during normal operation so if its pulling the battery down too far or the battery is failing, has a bad cell then you will see something like this. The truck may drive ok, but when you turn on hydraulics it pulls it down so much it takes voltage away from the steering motor and it will fail to run or slow down to the point it is no longer efficient to carry its load.