I'd go with the reccomendations. You don't want to risk damage. You can put synthetic in both differentials.
Another suggestion would be to post this in the Help section for the appropriate generation as more of our technical members watch those areas to assist the rest of us.
Hi and a warm Carolina welcome to the best RAM ForumZ on the web. How about dropping by the Newbie Checkin and introducng yourself then posting a few photos of your truck when you get it? I'm glad to see you popped back by to get an answer for another question after never getting an answer for your first question on the brake issue you first brought up. Let us know if you need help making more of this site so we can see more of you around here.