Welcome to Stovebolt!
Even in the stock configuration, you should get 45-50 MPH at least. What size tires are you running?
Anyway, back to your original question ... You should check out our Tech Tip (In the Tech Tip section available in the menu bar at the top of this, and every, page) on this very subject:
Rear axle upgrade for Advance Design 3/4 and 1-ton trucksEspecially out in the Western States, the donor vehicle shouldn't be that hard to find. Or check around with your local rock crawler groups. I bet someone has a 4.10 gear set to swap for your 5 -whatever it is.
Glad to have you aboard. The great Minds at Stovebolt (there are some ... somewhere ...

) should have you in the passing lane in a jiffy!
While you're waiting for your new gear set to arrive, please go over your brakes and steering. Maybe install some seat belts ...

John