With all that you have done to the engine, other than NOS, gears are your only option. 4.56's will put you back to oem spec.s, so you should be able to cruise at 80 even w/a head wind, and they will make turning the 33's easier when you have them on.

If shipping used diffs is going to be to expensive, you might consider dropping down to a 28" tire, for street use. With 28's and your mod's, you shouldn't have any power problems.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed