front axle shooting oil

hi im new here. have a '04 tacoma 4x4 trd, ext cab, 67k miles, 3.4 eng, standard trans. few months ago i had to go about 100miles down the highway and when i got to the destination i noticed some oil leaking on the ground and the chassis had a bunch of oil from front to back. on further inspection i found the origin of the oil was from the differential vent tube located on the driver side fender under the hood. in years past ive noticed a few drops of oil on the steering box so aparently alittle oil here and there would flow out the top of the vent and down the hose and end up on the steering box but its never been more than a few drops. after i cleaned off the huge mess of oil i figured it was just a one time thing but i had to drive a few hundred miles again and it spooged alot of oil out the vent again. after i got home i check the differential oil level and determined it was about 4-6oz below the bottom of the fill flug. i dont understand why all of a sudden its shooting oil out but never did this in years past. is it simply the vent cap has went bad some how causing the diff to pressurize ? should the oil level be slightly below the fill plug ? if so i cant see why i never had this problem until recently