How do I tell what gear ratio I have in my rear end? 2005 Dodge Durango 2wd V6 3.7 Engine?

A ball park could be found by making a line on the driveshaft and raising the rear tires off the ground . mark the tire with a line straight down you then count each time the driveshaft mark points back to where you started with the mark perhaps aimed at a bolt or bracket. If it passed 3 times and was facing exactly the opposite dirrection when the tire mark returned to the original spot this would be 350 to 1 or close such as 355 to 1 etc. 4 times would be 400 to 1 and 2 would be 200 to 1

The manufacturer used to put a small tag on one of the differential cover bolts, but if one is not present call a dealer and give them the VIN and they can tell you.