Gear oil overflowing thru vent tube

[truewakeboarder]

i have a 2006 6.0 with 6 inch lift 35 inch tires 3.73 gears ATS diff cover with Royal Purple 75 140 gear oil. all of a sudden i have noticed gear oil puking out the vent. and this only happens on my way to work. down and up a river canyon hasn't happen going anywhere else. i have checked the hose its clean and the fluid is where it should be. any ideas to what might be causing this and only at the same part of my drive?

thanks ryan