im getting ready to start the search for a new (used) rear axle for my 97 f150 and i was just wondering if i could use a 9.75 if the final gear ratio is the same? i have a 8.8 with 3.55's and will probably stay with that but if i came across a 9.75 with 3.55's i was thinking i wanted that since it is a heavier duty axle and im running 35's

 

Driveshaft length may be different, not sure. I wouldn't worry about the 8.8
I've got 36s on my 92 F150. I did upgrade to a 4.56 gear. I highly recommend you change gears, too. A 4.11 would be fine, I went with a lower gear just because. I've also got an 84 Bronco with 39.5 Swampers and it has a 9 inch.