Re: Dana 30 breather fitting?
That hole may be threaded, if so it's likely a 1/8 NPT and you can get a pipe thread to hose "barb" from any hardware store that sells plumbing fittings, screw it in and run some tubing up as high as need be. I wouldn't try to fill the diff from there though, the flow rate is terrible since the tubing ID is small plus the dif level is only supposed to be to the bottom of the fillhole, if you overfill it'll come out the cover, out the vent, out the pinion seal,axel tubes, etc generally make a mess, plus you may draw contaminants back in past the over stressed seals.
Do add the vent though, as high as possible and with some sort of filter. K&N used to have a small airfilter with three tubing nipples on the bottom for a consolidated vent set up, might make a nice point for the front diff, trans and transfer case to all vent to.