Yes - it's the yoke that attaches to the driveshaft.
Running the diff low on gear
oil doesn't hurt the gear interface as much as it hurts the pinion bearings. I hope you didn't run her too low..
I don't know what rear end you have but both 8.8" & 9" Fords typically run between .008 and .012" of backlash. In terms of how many degrees of rotation that might be on the pinion / Good question - and I can't answer it.
What I can tell you is that - properly setup - the pinion will rotate only enough to tell you that there is "some" clearance between the pinion and ring gear. If you can rattle it back and forth a good distance - you have a problem.
Another thing to check is the backlash in different positions. Looking at one of the wheels, rotate it so the valve stem is at 12 O'Clock - check backlash. Now rotate to 3 O'Clock and check again. Go through the 6 and 9 O'clock positions and check backlash as well. It should be an identical amount of backlash all the way around. If not - it's time to pull her apart before she pulls herself apart..