Time out. We're all looking at the problem wrong. He's got 33's. I bet the speedometer gear was never replaced. Keepkool, you're probably doing about 80, but your speedometer is only reading 60. To fix that, you can re-gear you speedometer ($10 mod), or you can regear you axles ($1000 mod, if you don't do it yourself). The latter is the proper fix and will definitely be worth it, but it's costly, as you can see. To verify your actual speed, have one of your friends with an unmodified vehicle drvie in front of you and you follow him. Tell him to hit 60mph and hold steady. Then you'll see that 60 on his speedometer looks like 45 on yours. Lather, rinse, repeat as necessary.
Basically, your 4.0 shouldn't be going that slow, even with 33's unless you've got other problems (like a plugged catalytic converter). But if you can get up to 3500 or 4000 rpms in other gears, you don't have cat problems.
corey