Lots of diesel talk here... nothing about the OPs question. I'm curious as well. Had a friend that bought the 6.4 with the 6 speed and he didn't like it because of the gearing. It would drop into 4th pulling a grade, then slam into 2nd as speed dropped. For whatever reason, it would constantly skip 3rd when downshifting. Now, he only had the 3.73 gears because he couldn't find 4.10s anywhere nearby.
The 6.4 seems like a great powerhouse, it just needs the right trans to make it a very good vehicle. Hopefully the new 8 speed is the ticket, but we still await on topic answers.