1998 Chevy Astro vibrations

1998 Chevy Astro Van mileage: 63,000. I have an all wheel drive with a vibration at between 60 and 75 mph. It gets worse when I put the engine under load, and smoothes out when the load is decreased. I have balanced the tires, rotated them, had both drive shafts taken off and inspected, and am now being told by a mechanic that the transfer case might be causing the vibration, at least a 70% chance. His recommendation is to replace it with a new GM factory built. I have been told that some vibrations are never found, at least not very easily. What would you recommend?