Check everything boxed in yellow.

You should not have any looseness where the leaf spring attaches to anything. Ubolts attache it to the axle at the center of the spring while one end bolts to a fixed location on the frame at the front and a shackle at the rear of the vehicle. There should be solid rubber bushings in the eyes of the spring itself like in the second picture.

The Ujoint at the pinion is also boxed on the second picture. There should be almost no rotational movement and no vertical or horizontal movement.. Check the bolts and tabs that hold the ujoint in the driveshaft too.

 

Update, while driving and vibrating, I turned the over drive off and it stopped vibrating. Any thoughts?

 

A miss

 

That's a miss, more than likely due to blown plenum.

 

A friend told me it's probably the torque converter?

 

Unlikely. T/C still locks when O/D is disabled. You have a miss. You are showing the classic symptoms.