I did the 5 speed conversion where I swapped the transmission control module from a '03-04 Jeep Grand Cherokee into my Durango and got a second overdrive with a ratio of .67. The shift points are pretty much the same but at 65ish mph it shifts into 5th gear so that I am at 2k RPM at 70mph with 3.92 gears. The part number I used is 56044574ac. it cost $45 from eBay.

thanks to this guy's youtube video: