Rusty, I'm not too sure how you guys do things in the States, but things being what they are, it must be much the same as us. Here's some explanation that should get you most of the way.
Your axle length will depend on the rim offset. Rim offset is the position through the split centre (as if you're splitting an oreo cookie) at which the rim bolts onto the axle. if this is dead centre, this is called zero offset. If the split line is more to the outside, like with most modern alloy rims, that's known as a positive offset, and vice versa.
To measure the offset of your rim (it is usually cast or stamped on the rim as +25, -10, etc), you can simply place the tyre flat down like a pancake and measure from the floor up to the flange that bolts onto the hub. Then flip the wheel over and re-measure, divide the difference by two and that is your offset. It is very important to measure onto the correct face of the flange, so if the tyre is facing inside down , you measure to the underside of the flange and when the tyre is inside face up, you measure to the top of the flange.)
So if you have an existing axle with zero offset rims and want to fit rims with a positive offset of 35 mm, you need to add 70 mm to the axle's original length.
You can also simply place your tyres the required width apart and measure the distance between the two inside faces of the rims. That will also give you the axle length you want.
Then just specify the stud number and PCD and whether this will be bolt mounted or fitted with studs. You'll also need to specify the bolt or hole thread diameter. The PCD is if you drew and imaginary circle through all the holes and measured this diameter. To be sure, all tyre dealers should have a catalogue specifying the rim PCD, stud number bolt diameters, rim offset of each model car on the road. (It's not a good idea to fit spacers on off road vehicles to take any shortfall.)
In Africa we fit heavier axles, not because we necessarily carry heavier loads, but because the bearing size on heavier axles are larger and can handle the increased side load that comes with driving offroad. You guys also like larger tyres, which also place more stress on the bearings, so take this into account when you buy the axle.
You're welcome to PM me if you're still not sure.
