Differential low on fluid n noise, damaged?

YES !! I'll go along with that. I wish you had time to wait and call them in the morning for some further advice and recommendations, but......
You obviously have a good feel on what you're doing here. Watching the gauges is more than 90% know.

Likely!.....you'll end up exchanging a drop in. Thankfully you have stopped before catastrophic failure which would render your current drop in useless as a core.
Your front driver pinion rotates opposite of the rear most driver pinion. This is how they get the power divider to fit over the front driver. That reverse rotation gear system can sometimes be over stressed and fail prematurely. Little things like an accidental over....shall we say enthusiastic tug when coupling to a new trailer, or pushing a bit hard into the dock etc., can stress those components a bit as you are in a very low trans ratio. In other words, torque amplifier.

 

No one mentioned the “Carrier Bearing”……

 

The diff isn't yet "overheating", but it is warming. The carrier is a good thing to check because of the noise it can create when going out. I suppose if it is out, there could be some harmonic issues that can cause a temp increase....? Maybe, dunno for sure. But it costs nothing to have a look see.

 

Are you loaded/empty - ?

I haven’t hesitated to wrecker back to trustworthiness before.

 

He means pay the wrecker bill to a shop he trusts rather than let a strange shop work on it.

 

Just trying to help -

If you’re under a load that creates more stress.

I have towed my unit home before, but if you’re loaded then you need more help.