labtek

The rear end in my 2004 F150 4x4 is an 3.73/8.8 .

Replacement units are very difficult to find and even if I could find one, there's no guarantee that it would work any better than the one I already have.

3.73/4.75 units seem more plentiful. I presume that the 2 types are not swappable.

It's looking more probable that I will need to rebuild my unit with new pinion and ring gears.

I have a chance to buy an empty 3.73/4.75 rear end- can I put new 3.73/8.8 gears in it?

The advantage of rebuilding the 4.75 instead of my own is that I can do the rebuild myself with the help of a mechanic in my garage and then take it to a shop to swap them out instead of lying under the truck myself.

redfishtd

I don't have the answer to your question . I just finished servicing my 3.73 ratio limited slip 9.75 axle . The tags were right on the axle diff cover . I changed the pinion seal(mine was leaking) and pulled cover to change fluid .
What I learned was measure as found and mark alignment points . Use friction modifier if it is limited slip . Use special rtv for gear case seal as it resists the friction modifier . Use torque wrench on gear cover , mine was 25 pounds on cover ,19 on plug .
Pinion seal nut- measure and put it back exactly how you found it . Drive shaft the same . 76 pounds on drive shaft spline bolts 4 each 12mm spline .
used 3 quarts of 75w -140 synthetic on my 2005 f150 lariat. Stated that right on one tag of diff cover .
Good luck with yours .

labtek

Thanks for the post- good information.

Yes, of course- 9.75 and not 4.75...

I must have been thinking about another on-going project.

It seems that rear end work is an art that not all are capable of.