My Eco has the 3.55 rear end. I pulled my TT over 6000 loaded from Ohio west over the Rockies and to the Grand Canyon and back. Absolutely no issues. It climbed up 10,000+ foot Wolf Creek pass in 4th gear like it was a speed bump. I don't know why they even offer a 3.73, except that rear diff offers a lock-up option that the 3.55 doesn't. Trans temp stayed right at 195 through the whole trip except when climbing up to Wolf Creek, and then it went all the way up to 204 for about a minute.
Mileage has been great. 18+ on summer fuel for mixed driving, 22+ on steady highway, and we averaged over 11 mpg while towing the trailer at 65 mph.