I did this a little bit ago but like to keep the blog up to date on the changes. The truck came with 3.54 gearing which Bill changed to 3.73. When I picked up the truck it had 37 inch tires, and at 55mph (my preferred speed) the tach was only showing around 1400 rpm. Too low for cruising especially with a headwind or hilly terrain. After some research I decided to put in 4.56 gears.
Thankfully I didn't do the work. It involved the repair of one locker and the front wheel bearings were apparently very difficult to remove. The 4.56 gearing with 35" tires now gives me 1750rpm at 55 mph. Yes the rpm does get up there if I need to go 80, but it's so worth it on the bottom end. I can now crawl a lot slower in 4 low. The truck no longer downshifts on the highway either. Just set the cruise and go to sleep.
I finally found out what that middle gauge was for. When I drove away with the new gears the mechanic made sure I understood about breaking them in and not getting too hot. I was going to get a gun type thermometer to monitor the rear diff until I realized I already had a gauge that did it for me. This is a shot I took while randomly driving around AZ to get the gears broken in.
Soon after the install I left for the northern states for some summer exploring. When the axle started warming up some I'd just stop and let it cool down. In this shot I was trying to get somewhere before dark so got a bag of ice to help with the cooling process.
It didn't take long for the temps to stay down. Now I get excited when the gauge reads 160 which it only does on long extended road climbs. In this shot I was using creek water to cool everything down.
It's been a year and a half now since I did the gear change. I did get some high temps and had been smelling gear oil also. Turns out the fluid was low in the rear because for some crazy reason oil comes out of the air locker solenoid. It comes out at about a quart a year, but is still frustrating. I don't like smelling the oil from time to time or the fact that there may be a problem with the rear locker. Vent tube is clear and locker works fine.