Engine Vibration @ 40-45 mph

Hello, I own a 1995 Honda Passport (Isuzu Trooper) w/ 85,000 mi. I am expieriencing some pretty bad engine vibration at 40-45 mph to the point I must let off the gas to stop the vibration. If I gun the engine from a lower speed up past 50 mph the vibration is not nearly as bad as when I slowly accelerate up to speed. Any suggestions as to what's causing this? Thanks!