I guess I'm not sure if it was synthetic. It was some valvoline 80-90, that I've had stored in a wiper fluid bottle for like ten years. I figured i'd be servicing it in the next few months and would be chucking it anyway.
Driving the truck without the front driveshaft, to work today, it seemed signifacantly quieter, like maybe half the bearing noise is gone.
I'm having the U joint and cv joint or whatever the double one is called replace at Rocky mountain driveline.