Loud howl from the rear end?

Purchased this vehicle and drove it home 200 miles, no noise. Drove it around for about 50 miles and then drove it on freeway 50 miles. Loud howling noise started coming from the rear end. It got really loud when accelerating and would almost stop when coasting. Keeping an even speed, it would continue to howl. Really loud howl. Checked fluid and it is okay. Fluid seemed a little dirty / dark. I found a full rear end at salvage place for $300.00. I am thinking about just buying it and changing the whole thing as just having someone look at it to see what is wrong is like $200.00.
From your experience, what do you believe would be wrong with it and a ballpark figure if you could for repair vs replacement of entire axle.

Thanks,
Larry

PS: I have another question about the BCM as the door locks, the interior lights, tail and brake lights and turning signals all seemed messed up. Will send that question in a separate question. Wished I would have never bought this rig I am beginning to feel. Maybe I should sell it as is.