I'm betting your factory rims are a four on 4" bolt pattern, and you're looking to put some 5 on 4.5" rims on there.
1. You need a wider axle, and a more HD one. A 3500lb axle isn't much more expensive than a 2000lb axle.
2. To mount Jeep factory alloy rims with a small center hole, you need a wheel spacer (and that's after you get a new axle with a 5 on 4.5" bolt pattern).
This is a long read, but it is (in my humble opinion) the definitive thread related to building-up a little Harbor Freight 4' trailer. Read it through, it should answer all your questions (and some you didn't even know to ask). Lots of good info, with links to other builds and resources. There's even a table-of-contents in the first post. Read it before you even consider getting rid of what you've got; these little trailers can be built to take a beating.
http://www.jeepforum.com/forum/f27/...ype-trailer-ultimate-build-up-thread-1180456/
The pictures above are of my little modified HF 4' trailer. Suspension has been swapped out for longer/softer springs with a shackle at the back end (factory slipper springs are way too stiff), shocks, cooler tray up front, axle is a 52" wide 3500lb axle with electric brakes, tires are 235/75/15" on 15x7" Bassett racing rims (DOT compliant). Tub is just a diamond plate aluminum box with a hinged lid. Tongue was slightly extended with a 2" receiver up front and in the rear. We just took this trailer from TX to Inuvik, Canada (almost to the Arctic Ocean) and back to TX with no issues (about literally 10,000 miles). And that's a tent-cot mounted on the lid (think of it as a RTT).
Let me know if I can help you in any way.
