So I had Yukon 4.88's, front and back installed this week,
and I have put almost 500 miles on the truck since.
I followed the break-in procedure as outlined in the Yukon gear Install Manual, and was extra careful(read paranoid) not to overheat my new gear-sets for these first break-in miles.
Here's my question: Does driving in 2wd still engage the front diff, but not the axles?Was my front diff spinning during these 500 miles, breaking them in? Or do I need to drive in 4wd for the same break-in procedure and an additional 500 miles to mate the front gears? My truck is a 99 with ESOF.
It will surely take me long time to accumulate 500 miles in 4wd during the summer months. Any ideas, tips?

 

your front end is always turning just feel the diff cover when you stop after a drive next time.. it will be warm kind of sucks you have to be turning that thing 24/7 kills atleast a couple mpg

 

I'm willing to lose a few mpg's for the return of all the lost torque that the new 4.88's have given me. On the way to the gear installer(abt 400 miles) I got 16.7 mpg average. On the trip home I got 14.1 mpg. I drive a big truck, so gas mileage is not high up on my list of important things.
It's good to hear that I shouldn't have to run around in 4wd to break-in the front gear-set. I'll grab some new diff lube at Canadian Tire next trip out and swap out the old break-in oil.
Now I just gotta install the body lift, and decide between 37"x12.5 or 38"x14.5.
Then I'll be rollin' with the big dogs...

 

Make sure you use the gear lube recommended by Yukon, not necessarily recommended by Ford.

 

Here is what it says in the Yukon instructions that came with the gears:
"Oil
When filling the axle with oil, use a high quality name brand and fill the unit to the manufacturer’s
recommended capacity. Synthetic oil is recommended for most applications."

I used a synthetic Quaker State 75w140 with Ford Friction Mod in the rear, and Valvoline 75w90 synthetic in the front.
Should I be using something else?

 

if you have the stock ford differential, follow ford's specs. the gears don't care what oil is in there. it is mostly the carrier that determines what oil and additives to add.

 

All spiral bevel type gearing (both hypoid and amboid design) need an additive package on the lubricant to protect the gears from the extreme sliding forces generated when the spiral bevel makes tooth surface contact. Use only lubricant with "GL5" rating on the bottle or container.

 

That's fine, as long as you are using the stock Ford limited slip packs in the rear.