Rear driveline on ford expedition xlt

Not likely as the 4x4 has a transfer case.

Just bought an 01 expedition XLT from arizona (where it’s spent it’s whole life). It’s been kept up it’s whole life, although in OR the elevation is different and it’s cold. It was also transferred on a truck with its hood end pretty much facing down. Not sure if this can cause sediment or other stuff to collect. The car does an idling thing and has since it’s been pulled off the truck (up,down, up,down, shuts off if I don’t put it in neutral and keep a little gas) it just had o2 sensors and vacuum tubes (?) replaces before it was hauled here. It also sometimes seems to have somewhat of a “hiss” or possibly(!!) a decompression sound that can be heard from my seat when I first start up or upon shut off. Any truck gurus out here have any ideas off the top of their head? I’ve read up on it possibly being and Idle intake valve but other than that I’m not sure. I basically almost brake checked a car behind me into my hitch at a round-about intersection and that’s the first time it’s shuts off while actually out on the road. I would not be able to let thing warm up on a cold day if I wanted to, at least not without sitting in it revving it every few seconds. RPMs don’t seem to go too high. If this is a Intake valve, what are the damages that can be done if I continue to minimally use as-needed in this condition? About 150xxx miles on the truck. It truly is in great shape for its age, although it sure would suck to run its health into the ground.