dragonmaster1369, Running these old trucks at more than 60 mph without modification is asking for trouble. To achive highway speeds, adding taller tires, an overdrive trans, or changing the axle ratio is the only way to get to get the engine RPM's low enough to avoid damage.
The 4.56 gear ratio limits these trucks to a slow top speed. No one, repeat, no one in the civilian world in their right mind went down to their local GM dealership and ordered a K30 with the 6.2 diesel and a 4.56 axle ratio unless they had a specific application for it. If you plan on running your truck at a consistant high RPM, replace the harmonic balancer and avoid scattering your crankshaft and other related parts all over the highway.