Remembered a thread with pictures of a later model suburban C20 driveshaft.
https://www.gmsquarebody.com/threads/carrier-center-driveshaft-bearing-removal.40561/
I think the problem is solved now.
The bearings in the carriers are often sealed ones or have a dust shield around them in the rubber and are greased.
I think the side with the thick rubber on the second picture should look toward the transmission. Facing the driving direction more dirt and water is thrown to this side.
Welcome from Austria. Damn that sucks. As the tires are intact I think everything under the car is ok. Get a second and build a new one out of the 2 and sell the parts you don't need. Or part this one out and get a new Squarebody.
Not much cheaper than in Europe, you can get it this kit on sale for ~650€
I have DeWalt tools since ~13 years, they work great.
As soon as you have their battery system you are trapped in this tool universe.
You can install the sender in the outgoing cooler line if you don't want to pull and drill the pan. There is a lot of debate where to put the sending unit, don't want to jump into that rabbit hole. You just need a fitting/ manifold to mount the sending unit...