Looking at the Weller page David posted they only list two 2-speed gear sets, one with 6.50 and one with 7.17 high gears, nothing better that you have now.
Your 8.25x20 tires are kind of small for a 2-ton truck, have you thought about stepping up to a 9.00 or 10.00x20? If you end up with 22.5 rims remember the tire sizes are not equal with the 20's, they are one number size bigger, but are the same diameter.
8.25 x 20 = 9.00 x 22.5
9.00 x 20 = 10.00 x 22.5
10.00 x 20 = 11.00 x 22.5
Mike B
