is it on the bottom of the tube? can you draw an arrow to it in the last pic for a better idea where it is? breathers are the only hole your supposed to have & they are on top of the tube. is it leaking? id clean the area around it & weld a patch on it (edit-unless its the breather i just mentioned, i didnt think i'd have to say it). good excuse to go 1 ton.
there is supposed to be a barb screwed in there that the vent tube attaches too. yours must have come out or broken off. screw a new one in if you can. you might have to get whats left of the old on out first. get a set of easy outs. it will make getting the old one out easyer.
i couldnt tell where it was on the axle from the pic, but like i said there is only supposed to be one hole and thats the breather at the top of the axle. on my 9" and D60 they are interchangable. dont know if the 8.8 is the same. use the easy out like posted, use a tap to clean up the threads. i guess if it becomes a problem you can weld a round thick tube about 1" high around it like they do to protect air stems & other stuff, but it may never happen again