I am pretty sure it is 3/8 fine thread. When I changed the rearend in mine it was completely plugged. The new rearend is leak free.

 

Ok, well after looking at it more, half of the old plug is stuck in there, and i can not get it out, so i got a barbed fitting and hammered it in a little, and put a little bit of JB weld around it, followed by some silicone. not really that good of a fix, but i guess its better than having the plugged up leaky rear end. it feels like the truck drives better without the plugged up rear end also.