I think that might be the drain hose for the air conditioner, just by looking at where it located on the passenger side, and it would be an open ended hose hanging down like that one is, the radiator overflow hose would be up on the left front corner of the truck. So it would belong in that location for the air conditioner drain hose for it appears to be on the right rear of the engine near the transmission like that. My 78 Ranger Lariat has air conditioning and it has that hose also. That's just my thoughts on that but I hope that you find this information helpful to you,
I think that's the vent from either the t-case or trans. Theres a couple of long lines on top of your trans that it hooks to. It should be secured by one of the bell housing bolts.