You guys are so full of good information.
I am planning the axle swap for other reasons also. A mechanic friend and I are doing an axle flip in the rear and lowering springs in the front to drop the whole truck 4" in the rear and 2"-2.5" in the front. I have to dismantle the driveshaft and rear leafs to do this anyhow so it was a good opportunity to just swap in a new rearend with the taller gears and not have to try and just change the internals. This will effectively lower the stance of the truck and level it out or possibly leave a slight forward rake. The Ranger axles from '92 back to '84 are 1" shorter on both sides. By lowering the rear 4" and having 1"shorter half shafts it puts the widest part of the wheeltub lower and should allow me room for full rear skirts.
Because it is a somewhat "permanent" mod I was trying to figure out what kind of FE gain might be had. I can still get a '84-'92 rear with the 3:73's. I just saw it as an opportunity to go for a little FE boost. I could actually get a rear with 3:27's but I think that might be too much on hills and in town. I mean, I am driving a truck with about 110HP.
In my mind I was thinking that if the taller gears didn't hurt too much that someday I might try the 3:31's (either tires or gears) because they make a speedo gear for that and I could keep everything in sync. Dare to dream.
If you all have anymore thoughts I am all ears.