There are lots of options for axles on a UA vehicle. We like to consider our tire sizes and work backwards, with axles being stronger than engines. One-ton diffs like Dana 60s and GM 14-bolts rarely have issues on the trip, but they are heavy and can be referred to as anchors and plows depending on the terrain. This year’s Spidertrax axles and Truhi-9 differentials worked great, but a low-pinion 9-inch axle isn’t bad (though it does get the driveshaft considerably lower). Dana 44s and Toyota diffs work well up to 37-inch tires. Dana 30s and Samurai axles have attended UAs on many occasions, but without skilled drivers these axles have suffered. We wouldn’t recommend them if you’re building from scratch. Also, realize that vehicles with lightweight axles need to pack light to keep the parts alive; weight kills.
Lockers and gearing are pretty standard suggestions. You need them both, and gearing should be low enough for your tire and engine size. Welded diffs and spools are dirt-simple and have been on many trips without fail, though they will chirp tires on corners and can cause understeer on the road. Selectable lockers are great on the road because you can unlock them, but they add complexity with their activation systems, so route your air lines and activation systems to protect them from heat and snagging trail debris. Auto lockers such as Grizzlies and Detroits are tough and simple, but even tire pressure must be considered for good on-road driving, and these lockers can fail from shock load if an axleshaft breaks.
Speaking of axleshafts, spare shafts and joints are good parts to bring along, as is a pinion yoke if your pinion is low, plus the proper socket for changing the yoke.
The UA covers around a thousand miles of road and trail over a week, and having the right powertrain of engine, transmission, and transfer case is important. We feel these parts need to be healthy and reliable, but not necessarily crazy.
Starting with engines, it needs to run cool. A rowdy 800-1,000hp big-block is cool, but it won’t always run cool on a hot humid day of rockcrawling. Four-cylinders, V6s, I-6s, and small-block V8s seems to work best. Leave the nitrous and lumpy cams at home; you just need something reliable—and tough. Diesel engines have survived many UAs, but they have three pitfalls: weight, noise, and healthy torque that can easily break parts downstream. Loud exhaust is cool in the mud, but can get old on long road days and when you’re trying to hear your co-driver’s spotting suggestions. Fuel delivery can be a problem. We like in-tank fuel pumps to keep the pump cool and happy, and shielded fuel lines to deter vapor lock.
Both automatic and manual transmissions have shined and failed on the trip. If you’re running a manual be sure your clutch is healthy. If you’re running an automatic transmission be sure you have a big enough cooler.
Low transfer case gearing allows better off-road control and reduces abuse of drivetrain parts. The Atlas transfer case by Advance Adapters has worked great on many UAs, as has the Offroad Design Doubler 203/205 transfer case. This is due to their ability for gearing lower than 3.00:1 in low range.
Be sure you protect all of your drivetrain with adequate skidplates. Even an iron transmission can succumb to a serious hit on a big rock.
Steering is the Achilles’ heel of many an off-road vehicle. A steering problem can ruin the trip fast. We have found the best systems to be a simple ram assist, where a ram attached to the axle and tie rod helps turn big tires. One problem we’ve encountered is when the hydraulic lines hit the frame at full stuff of the suspension, so be sure to check this prior to the trip. The full hydraulic setups are finicky and not always safe for street use. A standard power steering setup isn’t bad, but all the stress will be transmitted to the box itself, and broken steering pitman arms and sector shafts have happened in the past with standard steering systems.
Three suggestions: (1) Run a cooler for any power steering system; (2) bring spare hoses and/or rebuildable high-pressure fittings; (3) raise your steering linkage and tie rod up if possible to protect it from rocks. Bringing a spare steering box isn’t really worth the added weight, but be sure you know what model box you need if you need to source one locally.