Marco, you want to know about Chevy/GM, just ask me. I'm a GM geek, (besides working for them for 34 years). I'll do the best I can to answer the question.
The grease job zerk fitting number is about right. It should say or show the spots in your owners manual. You should be able to do this very easy maintenance item. Just takes a grease gun and some cheap rags.
The differential: Is yours a posi-trac? Does it have the G-80 option code. If it does, it's a posi. The diff fluid should have been changed initially at 5000mi. Then unless you tow regularly, at 75,000mi. It should have a good synthetic 75w-90 weight gear lube installed with either a G-80 or standard diff. Towing alot, it should be changed at 50,000mi intervals. Check your lube shop invoice to see what they used.
The trans. I wouldn't worry about until 50-60,000mi. unless you tow alot. You have a 4L80-E trans. The heaviest duty Hydramatic GM makes. There's one heavier duty, light duty truck trans. in GM's stable and it's the Allison 1000 5 & 6 spd. auto. It's used in GM's light & medium duty pickups with the 8.1 gas and the Duramax 6.6 diesel engines.
There's a new Hydramatic 6 spd. auto in a few current GM light dutys, but I won't get into it because it'll confuse you more.
Hope this helps a bit..........