when you enter the service operator menu system and go into the 'set travel and braking' menu if you do not see an option for setting the 'speed limit' then as you were told it was not installed 'as an option' when the truck was built.
If you want it to work you will have to pay to have the software updated to include this option.

If yale holds true to thier past methods it will require your service tech to call into the yale engineering dept and tell them what is to be done, the engineer will then give a code to enable that option or supply the tech with the correct module to load onto the VCM. It's most likely a software or firmware upgrade.