Check the outer boot on the axle, I bet it's ripped, and tossing grease everywhere.

 

That would be my guess as well. Grease from the CV axle. I'm hoping the angle that axle is at is because its up on a lift and at extreme droop, otherwise, no wonder it's leaking...lol....

 

looks like the inner boot may lack a seal at the outside bottom in the picture.

 

 

I overhauled the front diff in my 96 last spring, got a cannot pass up price on the rebuild kit..... which (whether front or rear) never come with the outer axle bearings or seals for some unknown reason....had to buy them separate which I did.... cant really call it "rebuilt" if you don't change em all...... there is a seal behind that flange that the inner joint bolts to, it is a common rear axle seal part number (can't remember though if it was the same PN as the axle seal as the rear end or not..... ) but given where the grease stain is at on your truck, I'm guessing outer axle boot too... used to be you could get CV boots reasonably enough, nowadays for the difference in price (not much) youre better to use that CV axle as a core and buy a reman..... on that truck left and right CV axle are the same.