chrysler town and country vibration at 39 mph and 58 mph. Is it the transmission?

What is your question?

My car makes a weird vibration when traveling at 38 or 39 mph, and again at 57 or 58 mph. What could be the problem? Could it be the transmission?

What makes this problem better or worse?

Vibration is temporary (Starts around 38mph) and stops when speed increases beyond 39mph and beyond 59mph

How long has your 2003 Chrysler Town & Country had this problem?

A month or more. Other mechanics don't know what it is.