64' C10 spraying rear gear oil ??

[john dob]

hi - i have a 64' C10 (292 4 spd, long bed) and drive it on the weekends as a work truck. last weekend after driving it i noticed a puddle a few inches accross of gear oil under the differental. i crawled under and there was gear oil on the rear, underside of bed and on the frame. the only place i think it could have come out of was a hole that i found on the front / passenger side of the rear housing (between the driveshaft and the axle tube) there was a hole in the center casting maybe 1/4" diamater. i thought this might be a vent hole but all of the vent holes i have seen were on the top of the axle tubes, this is not. did i loose a bolt or a fitting ? i appriciate any thoughts. thanks - john