I recently purchased a 2010 f150 with around 250k miles. The truck runs and drives like a brand new one except for one issue, the roaring and whining in the rear end. I changed out the fluid and have come to the conclusion my pinion bearing is the culprit. My question is, do I have to replace the 8.8 3.55 gear ratio rear with the exact same kind or can I use the 9.75 rear end with no issues. I dont want to put a different one on it if it will tear up my transmission.

 

The 9.75 is not an issue. The gear ratio has to stay the same if your truck is a 4x4. If not, then you can change the ratio as well. If anything I'd go numerically higher (3.73) not lower. I've read a few cases of 8.8's and 9.75's being swapped but I don't know if it's a direct bolt-in or there's some ancillary hardware that has to be swapped also.