Is the distributor loose? Was the new carb "tuned" by someone when installed? Was the timing messed with?
Did you put new gas caps on? possibly creating a vacuum in the tanks?
Bigger cams do have less vacuum. Closer intake/exhaust valves opening/closing. I've heard roller cams can help with this but I have no experience with it.
Welcome to the site. I don't know, that truck might be too nice for retirement. I'd be happy to look for an old beater truck and swap it with yours. ;):33:
Burned out bulb on the front side marker would be my first check. Then check the grounds. Lights work, fuse is good. Stop/Tail/Turn is one fuse. Have you installed any new lights? or something that needs power?
You can download the 1978 wiring diagram here...
Welcome to the site. Nice purchase from your uncle. It does look like a pinion seal, but be careful, engine oil leaks can drip down onto the front axle.
I would buy some Gunk engine degreaser and spray the engine and front axle. (be careful not to soak electrical connections or distributor...
It may look short, but how long is the slip yoke? Because if its 4" long and only 2" sticking out it's fine.
That being said I think the normal slip yoke is about 3". So, on a really hard brake it could pop out.
Here you go. 2 minute search. It has all your questions answered.
http://www.billavista.com/tech/Articles/14-Bolt_Bible/index.html
https://www.pirate4x4.com/tech/billavista/14b_bible/14b.pdf
I would suggest downloading, copying, or printing for future reference.
How much fuel was in the tank? If it was low, FI models are known to have an in-tank pump fail and start back up again. Pump can get hot and quit. After it cools run fine for a while again.
Also, check tank grounds for corrosion.
Just about there... 1st pic is close, 2nd pic needs to be more centered. You will have a slight noised after about 10k miles.
Yukon Gear has a good tutorial.
https://www.yukongear.com/blogs/how-to-create-read-ring-gear-patterns_1
If you have factory AC, then a vacuum noise of a hiss or possibly a squeal from a cracked line/connection.
As @PrairieDrifter said sounds about right. Early SB didn't have an off switch for the fan.
Disconnect the power to the fan from inside the engine compartment and see if noise goes...