I haven't had the need to do a wider axle because both my current military trailer project and the Jeep-tub trailer project required narrower frames than 48".
But if you wanted to keep the 48" width and do a wider axle, here's one for $112.88: http://kmtparts.com/axles-spring-mou...55-4-drop.html
. You specify the hub face distance and the spring center distance and they make up the axle for you to those dimensions. The one linked to is a 2000-lb axle with a 5 on 4.5 bolt pattern (same as the HF axle, same as the TJ and YJ Wranglers). You could use the stock HF springs, and probably the u-bolts and spring plates, although that would depend on the exact diameter of the axle, they don't mention the diameter on their web site.
They also make up 3500-lb. axles for $122.99.
Be careful when ordering axles. The one linked above looks like a "dropped" axle (at least in the picture and link name) but the write-up says it's a straight axle. It threw me for a minute. Unless you're looking for a lowrider trailer, you want an axle that's straight across. If you google "utility trailer axles" you'll get plenty of hits. Make sure you order by the hubface-to-hubface measurement/width and not overall length, and to modify a famous quote: "Measure twice, order once". Once you order it (especially if they weld on the spring perches) it's yours to keep.
$112 for a 1.75" diameter axle custom length including hubs is a very good price (thanks for the link Jeff). And if a 2.375" diameter 3500lb axle was only $10 more - well that's an easy call. Plus they'll weld on the spring perches? Now it's pretty much a no-brainer. Just need to make sure there's not a size limit on tire/wheel size for the included hubs.
And very interestingly (to me anyways) on the 3500lb axle you have the option of a 5 on 5.0" bolt pattern hub (I'm pretty sure that's what the JK's bolt pattern is - you can get matching rims for your trailer).
For example, here's a 2700 lb axle (there are tons of options out there) but you still need to order hubs:
Factory Jeep wheels have a deep offset. Based on some measurements I've done earlier, to run some 235/75/15" tires on Jeep factory steel rims on a 48" wide trailer you're going to need a hubface to hubface axle around 64-65" wide (of course, make sure YOU measure before ordering any axle).
Having said all that, just out of curiosity John, what's the measurement standing at the back of your Jeep JK from the center of one rear tire to the center of the other rear tire? On my TJ with 33x11.5x16" tires on 16x8 steel rims is 62". My trailer's tires center-to-center is about 51.5".