Nuts & Bolts: Cranky 9-Inch rear
I have a 1979 F-150 Ranger short box, and it is in pure stock form. It has a 351m, C6, 33x12.50s, and 3.50 stock axles. The axles are the reason I’m asking for advice. I believe the rear locker is failing. I have not pulled the third member out yet, but it is locking and unlocking the rear tires turning at low speeds on the street. I assume the front is probably an open diff, but I don't know. So without wanting to spend money on these parts yet, it looks like it is time to get into an axle build. I am building this truck as a street truck that I want to be at least capable off-road. I am also the kind of person that wants to spend money on parts once. I am going to crack the rear first.
What streetable locker do you recommend for the 9-inch/Dana 44 combo that is not selectable? Should I replace the axles with higher-spline chromoly units now? What gear ratio do you recommend for 33s to 35s? This truck will probably sit on 33s for several years so I am reluctant to go to 4.11. I hate having to throw thousands at axle builds, but with a clacking rear the time is now. Please let me know what you would recommend as I don't want to waste money overbuilding but I definitely don't want to take these axles apart several times over the years.
First off, if your truck is stock, then it has either an open differential or a Trak-Lok limited-slip rear differential, not a locker. Both worn-out limited-slips and broken-open differentials can make the noises you describe, so step one is figuring out which rear differential you have. You can decode the tag on the door, but the surefire way to know is to yank the third member and have a look, especially since you know something is wrong anyway.
If the gearset looks good, more than likely the problem is in the differential itself. You can rebuild both an open differential and a limited-slip very inexpensively, so if your main priority is being able to use the truck, then fixing what’s there is going to be the cheapest and easiest option. Here is a great-step-by-step Trak-Lok rebuild article you can reference if you need it.
If you find more damage than wear, then the best answers to your questions depend on what you plan to ultimately do with the truck. We generally recommend driver-selectable lockers like an ARB (arbusa.com) for dual-purpose street/trail rigs, but since you say you’ll be using the truck primarily on the street and don’t want a selectable locker, then adding limited-slips to the front and rear axles may make more sense than nonselectable lockers. Automatic locking differentials are great for off-road use, but they do have their quirks on the street and some people don’t really care for the occasional popping and banging from an automatic locker. The stock Ford unit in the rear is OK, but if it’s damaged or not present, we’d recommend going with a tighter aftermarket unit like a Detroit Truetrac (eaton.com) or an Auburn (auburngear.com) limited-slip. Both will enhance traction off-road but behave properly on the street, and adding one to the Dana 44 frontend will make a noticeable difference in off-road capability over an open front differential. Your truck should have 31-spline rear axleshafts, which is just fine for 33s and a tight limited-slip. We wouldn’t worry about upgrading to chromoly rear shafts unless you find one of the shafts is twisted or otherwise damaged when you tear the rearend down. If you find both differential and axleshaft damage, you might consider going to 35-spline shafts for further bulletproofing, but it’s not essential. Upgrading the front shafts to chromoly is a tougher call since we’ve popped many, many stock Dana 44 front shafts, but those were all with a locker and bigger tires. Chromoly shafts have gotten more affordable in recent years, and we would have a tough time not being able to justify upgrading if we had the axle apart anyway. If only mild off-road use is in the cards, then the stock stuff will probably serve you well for many years.
As for the gear ratio change, that also depends on your ultimate goals. You know the current acceleration and performance of the truck now, so if you want more acceleration off the line at the expense of some higher rpm on the highway, then 4.10s would be a good choice. We ran 4.10s and 33s for a long time in a Blazer and found that the benefits of the lower gearing outweighed the mileage reduction, but your situation may be different. Again, you may find nothing more than a worn-out limited-slip or open diff, and if that’s the case, your best bet might be fixing it and giving yourself more time to decide what you want to do and how you want to upgrade. If you do change gear ratios, note that you will need a different carrier for the front differential, as lower (numerically higher) gears will not work on the one you have, so factor that into your overall cost.