1974 f100 rear diff help needed

1974 f100, 302 auto

Just had the rear ujoints replaced about a week ago, I noticed when I drove it long distances, after coming to a stop and getting out, I would smell a rancid burning smell. Finally dawned on me it was the gear oil in the rear differential, or what was left of it. Just before I stopped driving it it started howling at highway speeds. (Around 70 mph and up it would kinda sound high pitched uluntil I completely let off the gas, it got louder if I slowly took y foot off the pedal)

The pinion seal was completely crud, it would leak severely when you tried to put oil back in, so had that replaced today. And everything sounds good.

My question is though, how hot should a rear differential be to the touch? Mine is hot enough to place your hand onto it, but just hot enough that you don’t wanna leave it on too long.

I’ve read that’s normal, and since I’ve never encountered any of these issues before I would like to be educated.