GC, your truck will have to build up some speed prior to the EB kicking in. I drive a similar road to work in the mornings and my EB won't kick in unless I build speed in excess of 40 mph. From that point, the TH and EB take care of the braking chores as designed.
Hmmm, that hasn't been the case with my truck. It will engage well below 40 mph when it's working. But like I said, at 60 mph exiting the freeway this morning, nothing. The behavior this morning seems more an issue with the Tow Haul setting not kicking in and causing the trans to downshift. The trans stayed in 6th and the rpm's just dropped to idle.