You have a Dana 80 rear axle and a Dana 60 front axle.
They both require synthetic gear lube.
Ford recently advised their techs to refill the Dana 80 rear axle with synthetic 75w140 gear lube.
For the front you can get some argument. Your Owner's Guide says synthetic 75w90. The current workshop manual says:
SAE 90 Premium Rear Axle Lubricant, Ford part number C2AZ-19580-F, Ford specification ESW-M2C105-A
I'm not positive, but I'm pretty sure that's a synthetic gear lube.
If it were mine, I'd for sure use synthetic gear lube, but whether it was 75w90 or 90w wouldn't matter much.
The technique described in the Workshop manual is identical for both diffs.
[*] Remove all but one top bolt. Loosen that one bolt but don't take it out yet.
[*] The gasket is a silicon RTV stuff, so use a putty knife or flat-blade screwdriver and break the seal near the bottom of the diff. It will make a big mess when draining if you're not carefull. [img]/ubbthreads/images/graemlins/cool.gif[/img]
[*] After it drains good, remove the last bolt and the cover.
[*] Use clean rags to mop all the lube out of the bottom of the diff. Inspect the visible parts for excess wear. Clean up the mating surfaces to get rid of all the old RTF. Use brake cleaner to get both mating surfaces squeeky clean, but don't let any get into the diff.
[*] Use high-temp silicon RTV gasket maker and put a bead of RTV on the mating surface and around all the holes on the diff cover.
Put two bolts through the cover at the top of the cover, at about 12 o'clock and 8 o'clock, and start those two bolts without messing up your nice bead of RTV. Then push the cover to the diff and finger tighten those two bolts. Then finger tighten all the other bolts in the diff cover.
[*] Tighten the bolts alternately and evenly to 35 lb-ft.
[*] Allow at least 1 hour cure time before filling the axle with the correct amount of specified lubricant.
If you are going to add friction modifier for limited slip axles, then add it after you have at least one quart of lube poured in.
Note: This procedure requires you to use RTV to make your own gasket. But if that sounds like a pain, then you can buy regular diff cover gaskets at most auto parts stores. [img]/ubbthreads/images/graemlins/warmsmile.gif[/img]