[jps4jeep]

Your axle vent line should not be attached or connected to anything other than the axle. i don't have a TJ, But the only lines that I could think of going from your axle to anywhere else would be the Diff breather (which in mine goes up to the fire wall) and the break lines. no other hoses unless you have air lockers (which i am gonna assume you don't but if you do, thats what it is) The lite and hiss, suction sound (again unless you have ARB air lockers) has to be unrelated to this mystery hose...

Are you sure that it originally came from the differential? or are you guessing that is where it came from cause when you noticed it thats where it was hanging?

It might be a vacumn line that pulled free and was hanging low near the diff, and a leaky vacumn line could cause the motor to seam like it is skipping, but not neccesarily in a MFI motor. ???

I have way too many questions, if you could, snap a shot of everything and post some pics, might help me a bit!