Awful_Economics

I just bought a '88 22RE RWD Pickup with 136k miles on it. Engine runs well, completed a rather in depth tuneup on it and all is purring along nicely. However I'm experiencing a noise while driving that I think is probably the differential..

The Facts:

• Sounds like a wurring, grinding noise.
• Noise is only present when moving AND in gear.
• Noise increases when throttle is applied but NOT when in neutral. Engine revs fine in neutral and no noise present.
• Transmission shifts pretty smoothly and through all gears.

Steps Taken So Far:

• Removed drive shaft from the differential and inspected pinon nut. Appears to be OK and tight.
• Replaced Diff fluid. Very fine metal shavings attached to plug, but very very minimal in amount. Like, I had to pick it out with a tooth pick.
• Backlash in the diff is present, maybe 1/4" in either direction?
• Jacked up rear, put truck in neutral and spun rear wheels. Can for sure hear bearing noise but I'm not 100% sure it's what I'm hearing on the road.

I feel like it's probably in the diff. Should I even consider doing a rebuild myself or should I source a used one and just replace it?

I'm never done any differential work before so I'm not sure what constitutes normal and what's not.

VIDEO LINK: https://www.dropbox.com/s/8aeh1k8k53..._0375.MOV?dl=0