Could someone tell me the mechanical difference's between a Ford 8.8 axle and a ford 9" axle?
Thanks

the ring gear is bigger on the 9" and the 9 has a 3rd member the 9" is stronger in my opinion by far

Yep, 0.2" bigger (that should've been obvious).

Other differences,
the 8.8 has a much bigger pinion shaft diameter, spline & main bearing (1.62" vs. 1.37").
The 9's pinion head remains bigger with deeper ratios (and begins to resemble a worm gear. This is due to the 9's greater hypoid offset)
8.8 has lower internal friction (less HP loss, less heat)
9 has the removable 3rd member, 8.8 uses an integral housing.
9 has pressed-on axle bearings, 8.8 uses c-clip retention.
9 has a 3rd pinion nose bearing (needed due to the very close spacing of the main bearings)

Both have 31 spline shafts (fullsize F-150/Bronco versions)
The ring gears on both are similarly thick from OD to ID, with wide teeth (by comparison, Dana gears have much narrower teeth).

Weaknesses:
8.8" stock carrier is weak (a non-issue with a Detroit or other full-carrier locker).
8.8" axletubes can spin in the centersection (good idea to weld them at the center chunk)
9" 3rds sometimes crack at the 3rd bearing support and/or blow the pinion housing apart (nodular ones excepted, they have a big "N" embossed above the pinion).


Either axle should handle 37" tires fine on a 4000-4500lb rig.
The 8.8 makes a much easier swap on later rigs with an electronic speedo as it has the required sensor on it.
The 9 has more aftermarket upgrade parts available.