Stock YJ, TJ, XJ, and MJ axles are 60.5-inches wide from the wheel mounting surface (WMS). If your new tires are a lot larger than stock, you're probably going to hit suspension components and sheetmetal as the larger tires turn when steered or as the axles articulate. When having new axles built, it is a great time to increase your track width not only to help keep the tires out of the chassis and body but to increase the vehicle's stability at angles.
How much wider you go is largely up to you, but John Currie says that, for example, increasing the front track width of a TJ 6 inches will allow 40-inch tires to be steered fully without hitting chassis components. Knowing what wheels you're going to be running will help you dial in your exact track width because you can measure your backspacing and have the axle built to accommodate it. If you have a set of aluminum rims with a 3 1/2-inch backspacing you'd have your axle built a lot narrower WMS-to-WMS than if you planned on running stock Hummer rims with 7 inches of backspacing because the wheels with less backspacing would make the tires stick out much farther past the WMS.
The Curries stressed keeping an eye open toward the future because no enthusiast ever considers his or her vehicle truly completed. When assembling an axle, it's not that big of a deal to drill the shafts and brakes for both 5-on-4.5-inch and 5-on-5.5-inch lug patterns because the components are being machined anyway. That way, if you ever decide to swap your XJ, MJ, TJ, Unimited, or ZJ over to the bigger lug pattern down the road, it will be a simple matter of knocking out the lug studs and reinstalling them for the larger pattern. Keep in mind, though, that you can't drill a dual pattern with a five-lug and a six-lug because some of the holes would overlap.
On a front axle assembly there's a lot to take into consideration. First, John Currie told us that most stock Jeeps have about 6 to 8 degrees of caster built into the axle. That allows the tires to self-center after a corner and to track straight going down the road. However, Currie has discovered that most Jeeps with larger tires actually drive better down the road with slightly less caster (about 4 1/2- to 5-degrees) because the larger tires tend to fight each other and want to induce shimmy. Therefore, it's better to know how big of a tire you're going to run and how big a lift you're going to install so your pinion angle can be set and the knuckles can be welded on with the right caster.
If you're going to have the axle builder weld on your spring perches or suspension brackets you're going to want to know your pinion angle, or at least how much lift you plan to run. Most axle builders can get the pinion angle close armed with only the amount of lift you're running, but it's best to get under your Jeep with a cheap angle finder and get a measurement. Take your reading from the face of the yoke and with the vehicle parked on a level surface.
Most major axle manufacturers have production lines designed to turn a decent volume of axles out per day. Asking for one-off or custom items like a full truss or custom bracketry means that, in many cases, the company has to stop its production line to complete your custom order. Most are fine with doing it, but you'll need to realize that it's going to add some cost to your axle. Having stock-type brackets moved a little one way or the other on the housing probably isn't going to be that big of a deal, but if you have the skills, it may be better to do your own highly customed or one-off suspension brackets.
Trussing is something that's best left to a professional shop because as the truss is welded on, the housing tends to distort in the direction of the weld. Currie often sees customers' housings with homemade trusses that have distorted the axle ends over 1/4-inch. The company has a lot of tricks up its sleeve to ensure the centerline of the carrier and housing ends remain in alignment when a full truss is added, from pre-bending the housing to account for warpage, to using an alignment bar to install the housing ends.
Don't try to test the manufacturer's warranty just to save a few bucks. If you're repeatedly destroying components with your big-block engine and 44-inch tires, most shop owners aren't dumb and won't buy your story that your ring-and-pinion exploded into tiny fragments when your Granny shifted your rig from Park to Drive in the church parking lot. If your setup requires a gnarly axle, just pop the extra dough for a gnarly axle that's going to survive. You'll save a lot of time and aggravation in the long run for both you and your axle builder.
Conversely, don't feel like you have to act all hard-core and buy the biggest, most expensive thing on the menu if you're not hard-core. If you aren't going to like the handling characteristics of a locker and your driving style and terrain don't require a big, high-zoot axle, then don't buy one.