What would I need to replace in order to fix this leak ?

 

Pinion seal, along with new crush sleeve.

 

Pinion seal but you don't h necessarily need to replace the crush sleeve

 

Like the others have said, pinion bearing seal. When replacing, be sure to get the pre-load correct.

 

Mark the position of the nut in the center of the pinion with a punch by making a dimply on the nut and then one on the flange in alignment with the nut dimple. Now remove the nut and count the number of turn to remove it. Remove the flange, R&R the seal, reinstall the flange. Now install the nut while counting the same number of turns it took to get it off until the two dimple marks you made line up. By doing it this way you avoid all of the crush sleeve and preload steps and cost.

 

Get an In/Lb torque wrench and measure the pinion preload prior to removal. Put it back together and copy that torque value. (with the tires and rotors off before you start). You can go a hair beyond the originally marked location if needed, like one degree or so to obtain the proper torque value without adversely affecting gear mesh. Doesn't hurt to check the gear mesh and lash afterward.
If the pinion seal has a spring tensioner on the rear of the seal hand pack vaseline or white grease around the spring prior to installation. This helps ensure the spring stays put thus holding in the oil. Use the proper driver or similar substitute to drive the seal in evenly. If you bend it, it won't seal. Inspect the seal surface on the pinion yoke. If you can catch your fingernail in the groove its shot. There is a repair...a speedy sleeve. But there is a trick to doing this properly. It's my guess that the yoke might be not a whole lot more money than a speedy sleeve? But if it is I can pass along the proper speedy sleeve repair method.
Then your next challenge is holding the yoke to obtain the torque spec on the nut. See where I'm goin. If all this intimidates you take it somewhere. $2-300 is way cheaper than a front axle IMO.