Help with Rear Axle

[PDXk5]

I have a 1972 4wd Blazer. The original axle has developed a leak where the axle tube is pressed into the differential housing. It sounds like it will be easiest to just replace the axle assembly rather than trying to have someone weld the seem ( from speaking with others here on the forum, it is possible but difficult to weld, as the tube is steel and the differential housing is cast iron).

Anyway, I was able to find an axle assembly here locally. I have purchased it and am now trying to figure out what exactly I have bought. According to the seller its out of a '72 4wd Blazer. I pulled the differential cover. The larger gear has 3 73 GM stamped on the edge. I haven't opened mine yet so am not sure what ratio I have. My axle is original to my rig and is equipped with Posi. I'm hoping to keep the rig with the same gear ratio and posi as it was originally.

With this in mind, I'm new to this and have a few questions, I'm hoping someone can help me with:

1.How do I tell if the 'replacement' rear end is a Posi unit?
If I spin the pinion both drums turn in the same direction. I thought this meant it was a posi unit. However, just spinning the pinion, sometime I would stop and when I started spinning again only one drum would spin. Stop and then start again, and then both would spin. Does this mean it's worn?

2. When I removed the diff cover I noticed a bunch of chunks of metal. One piece looked like a small spring and the others looked like gear teeth. I spun slowly the pinion and looked at all visible teeth in the differential and didn't notice any broken or missing teeth. What and where are these from?

3. I was wanting to upgrade the drums to the HD style that are 2 1/2 wide. I'm having a hard time finding an original OEM set up at a local salvage yard. Any suggestions? My understanding is the 11 X 2 1/2 drum brake was available from '77 to '91.

4. Shall I attempt to disassemble, reassemble and install or will it be more advisable to obtain all the parts and take it to a shop to put everything together and install on my rig? New bearings, seals, paint and installation... Any suggestions for a local shop? Any estimates on cost? I don't have to do this right away. My axle works great, it just leaks on my garage floor.

Thanks,
I appreciate your help.