If you tow even occasionally, you may want to keep the 3.73s. I have a 4.0 OHV with 3.27s. It gets great gas mileage, but it hurts it whenever I tow anything, even just a trailer with a mower on it. But best bet for you, if your set on 3.55s with LSD, you will have lots of luck finding an Explorer 8.8 in the junkyard. Most Explorer 8.8s will have LSD, but you will have to change the spring perches off of the bottom of the axle shaft and put them on top of the shaft for it to work properly on a Ranger and keep the back of the truck up. Not saying you won't be able to find a Ranger 8.8 axle with the specs that you want, it'll just be more common to see what you want in an Explorer rear axle.
-2011 F-150 Lariat: 5.0 "Coyote" V8, 6-spd Auto (6R80E), 3.73 ELocker. White Platinum Metallic Tri-coat.
-1992 Ford Ranger: 4.0L OHV, 5spd (M5OD-R1), 3.27 Open.