The 9.75 is a new kid on the Ford axle block. It debuted in 1997 and found a place in Ford's uprated-capacity 1/2-tons. Its primary home is in the new-generation F-150s with the higher GVW package or 5.4L V-8, but it can also be found under the '98-'02 Ford Expeditions and Lincoln Navigators. It's also been used in the limited-production 4x2 Ford SVT Lightning hot-rod pickups. There is an IRS version that replaced the beam axles in the '03-and-up SUVs.
Whether climbing, towing, or engaging in stoplight shenanigans, a strong rear axle is a key element to any 4x4. The Ford 9.75 is a beefy contender to the Dana 60 semi-floater crown, and we figured out how to make it stronger. | 
The 9.75 has a beefy centersection reminiscent of the big Ford 10.25/10.50 full-floaters. If this axle has a weak link, it's the tubes. While they are bigger in diameter than the Dana 60, they are fairly thin wall (.180-inch) compared to the D60's .250-inch... | 
...In strength, diameter counts for almost as much as wall thickness, but the Ford is still rated 500 to 1,000 pounds below the Dana 60's Gross Axle Weight (GAW) rating. Note the bead of welded material (arrow) that gives away the tube's HREW (Hot Rolled Electric Welded) construction. |
You can see the detailed axle specs in the nearby sidebar. It's a pretty beefy axle for a 1/2-ton. Big ring gear. Big axleshafts. Stout housing. In torque capacity, it's ahead of the legendary Dana 60, though it's a bit shy of the average semi-floating D60 in load-carrying ability. When you see it, you'll say, "Ah, a downsized Sterling 10.50!" and you'd be pretty close to grasping the concept. Other notable comparisons to the Dana 60 are that the 9.75 has about the same-size carrier bearings and larger pinion bearings.
The 9.75 has had a few changes in its relatively short life, including larger axle bearings and shafts in '99 (from a 1.620-inch inner diameter to 1.705 inches), a larger outer pinion bearing (increased from 1.31 x 2.68 inches to 1.31 x 3.00 inches), and the pinion was altered for a longer crush sleeve.
With the new-generation F-150s outnumbering everything but corner convenience stores, we thought it useful to look at this axle from a builder's/swapper's perspective and see what can be done to make it better. To that end, we enlisted the aid of the University of Northwestern Ohio's High Performance Drivetrains class to install just about every goodie currently available for the 9.75. This school is located in Lima, Ohio, and has a high-performance motorsports program beyond compare.
1. The 34-spline 9.75 may have one spline less and 0.070-inch less axleshaft diameter than the D60, but it makes up for it by having quality material. Ford uses a high-carbon SAE 1050 carbon steel in its axles instead of 1040. As a result, the Ford holds a significant edge over the Dana in short-term maximum output torque rating, and it's dead even in the maximum continuous rating. | 
2. With any axle, it's always a good idea to measure and record backlash and rotational torque before pulling the carrier for repairs or upgrades. That gives you a roadmap for proper reassembly. If these measurements change more than a skosh after reassembly, you'd better start looking for a problem... | 
...Total rotational torque is taken with the axleshafts removed but all other parts intact. Expect it to go up a skosh if you install new bearings and a new pinion seal. Note that the 9.75 uses a drive flange, another good design point. |
3. The new 9.75 Truetrac (left) and the OE open diff. The Truetrac has a very effective 3.5:1 bias ratio, but is almost seamless while delivering it. The Truetrac is a long-term option: No clutch plates to wear out, and no reversion back to an open diff as the factory Traction-Locks will do rather quickly. It's proven to be great in ice and snow because it doesn't have clutches that must "break away" or slip the tires when traction is low and the wheels differentiate in a turn. Rather, the Truetrac delivers smooth power all the way around a turn without breaking the tires loose. The Truetrac housing is also much stronger than the OE diff, so it also delivers in the beef department. | 
4. These beefy Yukon axles are made of 1541H alloy steel, which are only a couple of steps below chromoly in strength but much more reasonably priced. A 1541 axle is approximately 55 percent stronger than the usual 1040 axle material, and at least 15 percent stronger than the Ford OE 10.50. We also opted for Yukon's prepackaged wheel bearings and seal kit. Our '97 was the earlier unit with 1.625-inch-diameter bearing surfaces. | 
5. The TA Performance cover is a functional work of art. The two Allen-head load bolts have feet that support the carrier bearing caps between cap bolts. Under high torque loads, unsupported caps can distort. When that happens, the "pin" just got pulled on that axle's "grenade." In addition to adding a considerable amount of beef to the center housing, the TA Performance cover adds about an extra pint of oil capacity, helping the diff run cooler. The cover is complete with a drain plug and a high filler plug. We also ponied up for the optional bearing cap stud kit, which replaces the carrier bolts with aircraft-grade studs and nuts. |