Not sure what year your truck is, but the feed to the vacuum controls to the vacuum motors comes from a T shaped check valve on the passenger side under the hood. Have you actually checked the vacuum there with a vacuum gague. You need between 15 an 20 inches of vacuum at this point. A picture of one is attached to this post. The control inside the truck is a vacuum switching device, check the connection of the solid black vacuum line to the control. Pull the line and attach it to a vacuum gague or if you don't have one handy, atleast p[ut your finger over the end and see if you feel any vacuum. Gagues are inexpensive and can be purchased at say Auto Zone for under $20, and it is a geat tool to have.
If the vacuum is bad feeding the control, move out to the line on the bottom of the check valve. There should be a small vacuum block attached to the engine side of the firewall. One line should go to main vacuum, one to the transmission vacuum modulator (Automatic), and one to the bottom of the check valve. On the other side of the check valve where there are two connections, one goes to the vacuum reservoir and one goes to the vacuum control.
Keep in mind that the vacuum reservoir may be bad, if so you will have no vacuum to your control.
Good luck,
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