Backstory

Transmission was rebuilt at 86,000 miles 3 years ago (lost 5th gear). Sold to previous owner at 110,000 miles, I purchased at 125,000 miles 6 months ago.

I had the transmission fluid changed at the beginning on December. Here's a picture of the fluid that came out

A few weeks later, I heard some speed related scraping noises that went away after a few minutes. My first thought was wheel bearing.

On 12/26, I start hearing a whining noise, like a wheel bearing would make. It changes pitch and intensity as wheel speed increases, no changes on acceleration, deceleration, steering, gear change, or neutral. I was hoping it was a wheel bearing, but the fact that it didn't change when steering had me concerned. Here's a video of the noise

In the following days, the noise would come back after about 10-15 minutes of driving.

Took the car into my shop on 12/31 and had them check it out, the sound is coming from the center differential. On the way there, the noise didn't seem to come back while I was on the thruway, but when I got on the side roads, I could hear it again. The range and pitch of the noise/speed seems to have changed.

Diagnosing

I'm not sure exactly what is causing this noise. My main guess is the rear bearing on the transfer driver gear (output shaft) of the center differential. A lot of the people with center diff noise say that the noise is only during acceleration or deceleration, but that's not what my problem is.

Decision Time

They recommended that I purchase a used transmission and swap it out. Their rationale is that it's highly unlikely that one bearing is the only issue with the transmission, and that doing the service might end up being a waste of money.

I've been doing a bit of research online, and this seems like a somewhat common issue. People are writing up DIY center differential bearing replacement guides.

LegacyGT.com guide

NASIOC thread

The only used transmission that is in my area would cost $1,400, free delivery, 30 day warranty (parts only), and has 114,000 miles. Labor would be about $650.

If it's just the center differential bearings, parts would be about $175, labor about $400.

If the differential's viscous coupling is damaged, the cost would end up around $1,100.

To just open the center differential and see what it looks like would be about $250.

I'm leaning towards having the shop open up the differential and proceed from there. If it looks like there are worse problems then just the bearings, I'll purchase the local transmission, or order one with less miles and pay shipping, then have them install it.

Questions

What would you do if you were in my shoes?

With the noise being consistent with the wheel speed, and not changing depending on other factors, would that lead you to believe that it's the differential bearings, or the differential's viscous coupling? I would think if there's noise on load, that it's the viscous coupling, whereas if the noise happens all the time, it's a bearing.

Thanks in advance for any advice you can give!