My 97 4cyl automatic jeep wrangler has a pulsing vibration that starts at 50 mph.?

The pulsing vibration starts at 50 mph and stops around 60 mph. It only does it when I am holding a cruising speed or slowly accelerating between those speeds. When I let off the gas it stops. I had the transmission rebuilt and a new tourque converter installed a month ago. What could this problem be?
 
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