if you have military axles, they should be 4.56s
using a rpm calc
a 255/85-16 is just over 33in tall
your od will be .7
at 65mph rpm should be 2113 with 4.56s
very close to what my C30 turns with 4.10s and a shorter tire
http://www.csgnetwork.com/multirpmcalc.html
but i think with 4.10s or 3.73s you should be ok as well