8 3/4 Vent Blow-By

I'm having an annoying problem with my 8 3/4 blowing gear oil out of the vent when it's hot outside (80 degrees or hotter) and I run the car at speeds of 55-65 MPH or higher for extended periods. I've pumped oil out of the diff so it's about 1 inch below filler when cold, (car is on the ground/level), but it still blows gear oil out under the conditions I listed above. Seals are good at axles and pinion, no leaks. I used Mopar Sure Grip additive and NAPA Premium Performance Gear Oil SAE 80W-85W-90 (GL3, GL4, GL5) when I filled rear. Center section is not new but is a used one I had, it's got 3:55 gears and cone-type Sure Grip, rear end seems to work okay. Any ideas why this thing is puking oil out of vent?