I've also been down the other side of those hills in LOW (of 6) still on the interstate, and I'll tell you that really is no fun, 15 mph with traffic passing at 65, Wow!
Forcing the truck to move slow going down hill may be one of the most compelling arguments I've seen for a manual trans.
Though I think your suggestion (regarding going up in granny) of less load being the right answer is probably an even more appropriate solution when headed back down.
If you have to go that slow you're almost certainly overloaded.
