You don't say what year your pickup is, but almost everything made in the last 8 years or more, the engine/transmission computer interface senses several inputs as you start downgrade, you manually drop down one notch on the selector, or press a button on the dash, apply the brake at the speed you want to maintain for about 2 seconds, and the release brake pedal and inasmuch as the engine and transmission can, it will try to maintain the speed you selected. Think of it as setting a downhill cruise control. This of course doesn't work well with a 14,000 pound backhoe on a 6% grade. Owners manuals are so dumbed down that they don't explain this well, Toyota, Dodge and GM all have it, I don't know about Ford or Nissan.
