Looking for a good, complete final drive axle

Good afternoon everyone!
1999 FLD120 single screw.
Was quoted $5400 + shipping/tax to rebuild just the differential.
Just curious. Does anyone have a good final drive axle (entire unit) which would be a direct bolt on to replace this 53 year old (1973) axle housing/differential lying around they would be willing to part with?
I live in Utah and would be willing to travel for the right unit.
The truck is for personal use so I'm trying to keep the cost low.
The truck is air suspension.
Hit me up and maybe we can verify if it will fit or not.
My number is 970-424-4552

 

Inland is where they quoted $5500 just to rebuild the differential.
I wouldn't mind going back to a more modern axle assembly where all the parts are not discontinued and hard to find.

I use the truck maybe 10 times a year and trying to keep the cost low.
I would think a used unit would be cheaper than $5500

 

Thinking of going this route....

2008-2017 FREIGHTLINER CASCADIA 125 MERITOR REAR DRIVE AXLE RATIO 3.42 OEM | eBay

 

Anyone ever deal with Spartan truck parts in South Carolina?