i believe the part # you speak of is MD712012 Drivetrain : Transfer Case Output Shaft Seal. no mention of a transfer case vent in "the" mitsubishi manual. you also mentioned "Three different folks at different times have replaced the seal to no avail" so thoroughly inspect where seal seats into the case (may need sealant) due to scored case. also...is this a stock height truck?...as the truck height increases (or differential moves down & away from case) the driveshaft angle increases & is no longer fully inserted into case. also swop out fluid if viscosity is in question, transfer case lube is "GL-4 SAE80W or GL-4 SAE75W-85W" per "the" mitsubishi manual.