Differential clunk when going in drive

Been chasing down a vibration in my drive line for a few days now. I feel the vibration when accelerating or when on the go-pedal but not so much when just coasting.

The transmission has already been checked and was told there is nothing wrong.

Now, I notice a distinctive clunk/pop when shifting in drive - most notable when I have shifted from reverse to drive. After getting up underneath and watching the rear end, I can see the input on the differential jerk when I hear the noise.

I assumed universal joint and proceeded to disconnect the shaft in order to change out the joint. Before removing the joint, I wiggled the input plate and found that there is some play. Then did a little looking around and found that I have apparently been leaking differential oil from around the bottom of the cover.

So, I removed the fill plug and tried to see if I could feel any fluid with my finger. I couldn't.

Obviously, it is time for a new gasket and fluid change.....but is the clunk a sign of it being time for new gears as well?

I have never changed out a rear end before, so I have to ask if there are any good tutorial videos online (assuming the worse) and wondering where the best place to get the gears would be?

Thanks in advance