Differential Identification?

Hello guys.
I have a 1999 FLD120 and I've been trying to determine the source of a nasty vibration I have, but I'm pretty sure I found it.
A little more info, I don't think it's the original diff. The VIN says the truck is a T/A but it is only a single. Plus, it seems geared lower than what I would think it should be. 70 mph = 1800 rpm with an Eaton 10 speed.

The Yoke on the differential had a lot of play (radial) and would move 1/8" - 1/4" by hand. I undid the nut on the yoke and had no problem puling it off by hand and without tools. But another reason I don't think it's an original diff is the yoke still has the sticker on it with a date of 6-4-1991 and part number S2654609.

Big question is, how do identify this differential so I can insure I get the right yoke and tube shaft?

 

I did find the riveted tag but not sure if it is fully legible.
Also took 4 more photos but they don't seem to want to transfer so I can share them.
Hoping the tags gives enough information.

 

If I'm reading that tag right.......

This axle is from 1973?????

Is that what everyone else is seeing also?

 

I think I would have to disagree on the date. The 1 in the first position of the date doesn't look anything like what you see as 13 on the year. That number before the 3 has a curve to it. It doesn't look like any of the 1's on the tag and 9 is usually an upside down 6. Only number I can relate that curved stem to is a 7.

And besides, in 2002, Rockwell was defunct and rebranded as AxleTech. So, technically, it would be impossible for this Rockwell (per riveted tag) differential to be a 2013.

So, if it is a '73 model.... Anyone have a newer rear axle just lying around they want to sell which would replace this one? I would imagine parts are getting scarce for something this old, plus a little higher gear ratio wouldn't be bad either.

 

I'm leaning towards just getting a replacement axle instead of fixing something this old.
I'm in rural southern Utah and have a few friends with acres of broken equipment which might have one I could buy.
I'm just not sure a 53 year old diff is worth fixing, or if the parts are even still available.

Thoughts?