Welcome to Bulltear!!!!
Stock tach? If so, it's calibrated to a 6 cylinder (3 spark pulses per crank rotation), a V-8 has 4 pulses per crank rotation, so your tach would read 25% high. At 4500 RPMs, that's 1125 RPM high, so it would be turning about 3375 RPM.
If it's an aftermarket tach, there should be a setting inside for 4-6-8 cylinder.
That still doesn't sound right with 31" tires and 2.73 gears. My AMX with 26" tires and 3.73 gears turns about 3500 at 75 MPH. My CJ with 4.27 gears and 35" tires was 3200 at 75 MPH. Either you have much lower gears (high 4.xx or 5.xx) in your axles or you're in low range. 2.73s are considered "highway gears". Not much stump pulling power there.
If you do have low gears, then bigger tires is the only solution, unless you re-gear the axles or install an overdrive tranny.
The gear calculator here says you should only be turning about 1900 RPM at 65 MPH in 4th gear with 2.73 gears and 31" tires.
http://www.grimmjeeper.com/gears.html
