You're lucky. My vent was completely gone when I got the truck. Not sure how long it was gone for, but I found rocks in the diff when I opened it and the oil may as well been tarsand.
I used a brass barbed fitting and pounded it in the hole. I am going to do a simple hose up to the frame with a coil to keep the dirt out.
I've got that beat. Some previous owner put a pipe plug in place of the vent fitting. A guarantee of soaked brake shoes and gunk all over the differential.
The hole should be threaded with 1/8" pipe threads.
