If the problem is only at cruising speed in 5th its because the engine revs are at the right frequency to set up a resonance in the exhaust. If so you should find that it will be the same in any gear a those revs with a constant throttle, but because you are accelerating though the band in the lower gears it is not a issue. Adding a couple of resanators will most likly help if not fix the ploblem. Take a look under any car on the street you will see two mufflers, usually the last one is a resonator to fix this same problem.
Pete
