Most likely the rear tires were also 7.50 x 20 from the factory. The supply of 7.50 traction tires has all but dried up except for the NDT military tires so people began to use the more readily available 8.25 traction tires on the rear. If your trucks gearing and motor are currently happy with 8.25's and it's not lugging and it is a two wheel drive truck then a new set of 8.25x20's should be OK.
-Mike-
