Goto page 82 of that link I posted or page 92 of the PDF. Looks like you got an H150/H170 rear.

The best ratio is 5.83 and here you can find a few places that may have the ring and pinion set in stock.

http://truckpartsinventory.com/heavytruckparts/search/3869006

These are usually around $300 $450

I ran the numbers and if your tires are 36" tall and top gear is 1:1 you should run.

50mph @ 3400 RPM With 7.20 gears
50mph @ 2900 RPM With 6.17 gears
50mph @ 2750 RPM With 5.83 gears