Ford 8.8
The Ford 8.8-inch is basically a 7.5 with a larger ring gear. The only way to distinguish it from the smaller unit is to look at the vehicle it's under or inspect the housing for identifying numbers.
Applications: '81-'94 Bronco, '90-1/2-'96 Explorer, '90-1/2-'92 Ranger with 4.0L engine, '81-'90 F-100, '81-'96 F-150/F-250, '97 F-150
Ring-gear diameter: 8.8 in
Pinion-gear diameter/splines: 1.626/30 splines
Available gearsets: 3.08, 3.27, 3.31, 3.55, 3.73, 4.10, 4.33, 4.56, 4.88, and 5.14

Ford 9-Inch
Appearing in almost every light truck with Ford written on the tailgate, the Ford 9-inch is one of the strongest light-axle designs around. Though production ceased several years ago, these units are still plentiful in wrecking yards, not to mention from custom-axle builders. The 9-inch is easily spotted because it uses a third member, or dropout, to house the ring-and-pinion, making it accessible from the front rather than the rear of the axle. This means there is no bolt pattern to look for, only a round hump.
Applications: '66-'88 Bronco, '57-'87 F-100/F-150/F-250
Ring-gear diameter: 9 in
Pinion-gear diameter/splines: 1.313 in/28 and 31 splines
Available gearsets: 2.80, 2.86, 2.94, 3.00, 3.10, 3.15, 3.25, 3.40, 3.50, 3.55, 3.60, 3.64, 3.70, 3.75, 3.80, 3.82, 3.89, 3.91, 4.00, 4.11, 4.22, 4.30, 4.33, 4.40, 4.50, 4.56, 4.57, 4.63, 4.71, 4.78, 4.86, 5.00, 5.14, 5.22, 5.29, 5.43, 5.50, 5.57, 5.67, 5.83, 6.00, 6.14, 6.20, 6.33, and 6.50