Changing gears in rear end
 

I have a 2011 Dodge Diesel with a 6" lift and 37"x 13.50x22" tires and I'm looking at re-gearing in order to get my truck back to where it was before the lift or at least close. Could someone tell me how to find out what gear ratio I have now and what steps I should take to get the right gear ratio. Any info would be appreciated. Thanks