Is the 4.3 ratio specification for the base models with 17" tires?
If so then the models with 18" tires will have a different ratio.

Looking at a tire spec web site I found the following:
LT245/75/R17E = 32" diameter
LT275/70R18E = 33.5" diameter

Using the formula

Effective gear ratio = Old Tire Diameter/New Tire Diameter X Gear Ratio

32 / 33.5 X 4.3 = 4.10

So a F250 with 4.3 rear and 18" tires has an effective Rear End Ratio of 4.10

Am I right on this????

So if I order a 4.3 I'm really getting a 4.10