For what it's worth, I have a similar issue, it started after I money shifted from the top of 2nd to 3... wait, no, first. Overspun the engine I believe. No issues and nothing nasty in the oil, but I started to get a vibration when letting off the gas around this rev range in the lower gears usually.
1989 Mustang GT: 5.0, 5spd --- Gone, but never to be forgotten. SVO brakes, 331ci engine, Off road X pipe, MM suspension bits.
2012 Mustang: V6, PP, 6spd MT --- Bone stock daily driver and D Stock autocross car. Track days? We shall see.