Having the flexplate balance out of line with what the motor needs will cause a vibration. Fords are notorious for needing different balance flex plates/flywheels for different years or displacements of motors. The FE family has two different balances. 360/390 are balanced in a different spot...
Couple broken rigs on Rocky saddle this year. I had a TJ quit just past the burned area that we had to leave overnight and then tow out the next day...That was a bitch driving a dead rig out.