put in 8 3/4, now have vibration

How much of the slip is in the trans ? before we just go running out and get a new shaft and then find out that slip is 9" long and you didn't need a new shaft.

You said the vibe started after going to the new gear, so you already had this entire driveline in the car with no issues.

So what was the gear ratio in the old 8.75 ?

Seems to me you may have made a big enough jump that you just need your shaft balanced