Calculating Rear Diff ... check my math!

The torque converter is slipping some, so third gear is not 1:1 to the engine speed. Depending on the load on the torque converter the slip rate will vary. You probably have a 3.23. Assuming an open end diff try putting the car on jack stands, put it in neutral (engine off) and have one wheel held still. Then rotate and count the revolutions of the drive shaft while watching the spinning wheel rotate one revolution. A 3.23 should be 1.615 (just over 1 and half) revolutions of the driveshaft (3.23/2).