I just read another thread that states you have a cummins and a 5 speed(NV4500?) 65mph looks like about 1612 rpm. The diesel should be able to pull that on the flat.
I don't have much experience with larger rear ends, but I remeber the 1 ton dodge diesel I got my NV4500 out of had an 8 lug 3.55 dually rear. Perhaps you can make an 8 to 5 adapter.
