Try doing a good cleanup and run the truck at highway speed with the rear axle on some heavy-duty jack stands for a few minutes. You can watch the pinion flange that way and see exactly where the leak is coming from. The gear lube should be at the bottom of the fill hole with the truck on level ground. Did you remove the vent and make sure it was open all the way to the axle tube?
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!