I have a 2007 ranger 4.0 V6 4x4 and am trying to figure out what kind of diff fluid I need. The axle code on the door says R6, according to the manual that means it's an 8.8 LS 3.73. The ranger station says that axle was used from 09' to 11' though. And it also includes an "8.8 high torque" that takes a different fluid. Which do I have?

 

A quick Google search showed me 80-140 for these LSD applications and 75-140 for the "High Torque" application.
Reading here, and a friend's experience with his Ford, underscores the use of genuine Ford XL-3 Friction Modifier.
Another brand caused chatter in my friend's diff.
Given that many (most?...all?) synthetic gear oils are said to be pre-treated with FM, I opted for conventional gear oil and XL-3 just to keep the critical juice correct.

 

You don't have a high torque in a stock Ranger, maybe in a 5.0 Coyote Mustang, but not a Ranger.