Hose ?

[DMAX-HD]

What is that 1/4 hose that connects to the rear-pumpkin on the driver-side?

To claify it connects to the rear-differential and looks like a brake-line - is that what it is????

[DmaxMaverick]

Vent. It accomodates expansion/contraction of the lube due to heating and cooling cycles. It should route up to about the fuel filler pipe. Keep it from getting pinched or kinked, and a constant incline all the way up. If it has a low spot (like sink trap), it could accumulate fluid, and block it. Next, is a blown wheel seal.