question for you guys w/ 35 inch tires

[hilljob26]

i have a 72 4x4 w/ 35's,i think i need to change the speedometer gear in the transfer case,where exactly do i find one and what do i ask for,i took the truck on its first road trip to get the bed done at the body shop,and the speedometer just didn't feel like what i was really driving,like it said 40 when i felt like 50-55,or was it the other way around,oh well,can anyone help