As far as how to do the rear, look up a few different videos on youtube to get an understanding on how it should be done.

One thing I recommend that others will disagree on is buy yourself new bolts for the differential. They can be had cheap and dorman (Part No 960-107) makes them. I had the luxury of breaking one off when tightening down and having to drill it out. Mind you, I hadn't tugged down on it and it just broke I guess due to the age.

It's not an absolute requirement, but if you can afford $10 or less to do it, go for it.

Regarding viscosity, as mentioned above, 75w-90 if you aren't constantly towing, and 75w-140 if you are constantly towing. Use a good name brand full synthetic gear oil. Don't spring for the walmart supertech gear oil (dino, blend or syn), it's just not all that great. I know from experience!

Regarding RTV, there are many different colors, but I use the Red High Temperature RTV for my differential. Works like a charm for me!