Not unusual for the yoke to get seal cut. They make repair sleeves if it's deep.

But, could be leaking through the splines too. Or even around the outside of the seal if the housing has imperfections.

I wouldn't take it back to them if they missed these things. Good news is you can fix it yourself, there's no voodoo in a 742.

On a 68 charger, 8 3/4, where would the vent plug be?

They can weep oil down the pinion gear splines and past the nut and washer. Most of the time it is the seal, but that is another potential leak source that gets overlooked. When the whole area is full of lube mess it can be hard to tell sometimes.

Exactly , so where is it actually leaking from, behind the yoke shield or at the u joint ?

Hi Mopar , Did Shop use a Original style 742 MOPAR # 1828719 leather seal , or the rubber Mopar # 2404135 same as SKF or CR #18912 ? Need to know to answer . Ronnie

I have a similar situation. I took a Dana to the local ring & pinion shop back in 2015 to have new gears installed as well as axles and seals. Put the rear end in my car (which was in the process of being restored so it wasn't being driven yet) and in a very short time I noticed drips on the garage floor. Checked and found fluid leaking out of the pinion seal. I didn't have the spring perches welded on yet so I just tipped the rear end up so no more fluid would leak out. I still don't have the car back together yet but once I get it on the road I guess I'll have to take it to the pinion shop and get another seal installed.