transmission shifter and transfer case shifter boots
Your shifter boot could be torn allowing road dirt, grit and water to get into your transmission gear oil. It could also be allowing that nasty gear-oil smell to get into your cab. All of the little trees hanging off of your mirror won't get rid of that smell.
Easily replaced even from inside the truck without having to remove the trans.
Check the shape of your transmission shifter boot before ordering, some W-series used either shape.