No, the carriers are NOT the same.
the only common part between the 28-spline and 31-spline 8.8" rears are the limited slip clutch discs (if so equipped)
The bore for the "pilot" on the back side of the axle pinions is larger because the side gear itself is larger (to maintain "wall" thickness with a larger diameter axle shaft hole.
However the gear (the teeth) is smaller, while the "spider" pinions are larger in actual diameter.
You can talk about the 3/4" cross-shaft "Low-Torque" 31-spline 8.8" axle but it's
unlikely you will ever see one, they were never terribly common
and all over 20 years old.
AllanD