I was down greasing the rear u joint and notice I have a leak that is starting. Do you think I could just put some rtv over it and hope it seals?

 

Looks like rear pinion seal is bad.
You need a new rear seal

 

Clean it with simple green or soapy water and hit it with a pressure washer. Biggest reason diffs leak is due to overfilling, check the level.

If it keeps leaking, you need a new seal..... trip to the local ring and pinion shop for a rebuild.

 

X2