With your current setup (4.10ratio,,31.6tall tire,,1:1trans,,75mph)..your turning around 3270rpm at 75mph
Gear swap sounds like it's in order....or get a 4l80 trans and have the best of both worlds...low ratios and od....the od would get you down to 2289rpm at 75mph
Bad part of gear swap is cost x2 for a k series..
Heres a rpm calculator thatll help you....
http://www.csgnetwork.com/multirpmcalc.html