Blazer Speedo gears

[BBrendal]

Ok so I put the 33 x 12.50 x 15 BFGs on and love them. Now I need to get my speedo right. I havent checked with a gps how far off the speed is but it is registering 9/10 of a mile when I traveled a mile. So I know the speedometer is slower then the truck is actually going. I know someone who can figure out the gear combo that it has to be on a transmission but on a 4x4 how do you get the speedo gear out of the 205 transfercase? A socket does not fit on the housing in transfer case. Has any one put these tires on and changed the gears. Any help is appreciated. Oh and also Im running a 3.73 gear.