differential roaring at 45 mph

[Mako]

2000 espy 5.4 EB axle H9 lsd 3:55
It started 9 months ago, after I had the trans rebuilt along with the rear wheel bearings and seals (leaking at the time). I now have 143K mi on her and I see that many are having the same issues but I don't see any catastrophic failures as a result of not doing anything.
I am going to drain it and clean what I can out, inspect and add 75w140 synth, and 7oz of friction modifier.
Not sure the tag on the hub means but reads:
S 939 P
L55 9 75 OD 30
I see this is a common problem by searching here. Thanks for this helpful forum!

 

[FordandPolaris]

Best I can figure on the tag is that L=LSD, 55=3.55, 9 75=9.75in Ford rear end (Expy's came with either the 9.75 or 8.8) and OD for overdrive in the trans. Basically it just identifies exactly the kind of axle used in case it needs to be replaced.

Mine hums, my gma's explorer hummed from the day she bought it, and my dad's Expy also hums a little. If it is roaring, then it could be a problem but honestly until something catastrophic happens I would not worry about it. It's going to be more work than it is worth to fix something that isn't broken yet.