You can set the cruise at 35 MPH or higher, which is very inconvenient for those 25 MPH limit roads. However, I found out that once 35 MPH cruise is set, you can use the coast button to slow the cruise to 30 MPH. Once you coast to lower than 30 MPH it turns off. Unfortunately you can't resume to anything below 35 MPH.
I'm not sure why DC programmed these trucks to not be able to set the cruise below 35 MPH. My Honda Accord (5-speed manual) can have its cruise set as low as 25 MPH.