According to the book with a CV shaft your pinion should be in a straight line with the shaft so if it’s at 8 so should the pinion. That’s the starting point anyway, everyone seems to come to a different final angle on the pinion based on trial & error, I think my driveshaft is a degree or so steeper than the pinion, that was the sweet spot on my TJ. If you’ve tried a bunch of different angles are you sure your vibration is even coming from the rear shaft?
Also consider the impact of varying weight as it can change your pinion angle, for example when my TJ is in the garage it’s usually empty, the spare is off & the tank is close to empty but when I pack it up for a day in the mountains I add a toolbox at around 80lbs, a spare that’s around 100lbs plus a full tank of gas which adds close to another 100lbs, toss a cooler in & altogether that’s probably 300 extra pounds compressing the rear springs which is enough to drop the angle a degree or two. Oh, & my fat 140lb ass too.