Vibration at 45-55mph? Try this unexpected fix...
 

I have had the common 45-55mph vibration on my FB for the past 18 months. Same in acceleration, deceleration and cruise. Balanced my wheels/tires, driveshaft (even swapped mine with my parts car) to no avail. Had to remove my 81's oil cooler to fix a hairline crack and that's when I saw the left front engine mount broken. Ordered two new mounts and a new transmission mount, all vibrations problems fixed. That's two sets of front and three trans mounts in 29 years, guess that's not too bad overall.

Anyway, check the engine and transmission mounts. Mine were in bad shape, typical of the AZ heat.

thank you for the write up, My drive shaft broke on me, and i bought a used one, the u joints seem fine but it's still vibrating pretty bad, i'll have to check the mounts

mine vibrates at 75mph. all of that stuff is good on mine, i think its just saying "dont drive me any faster!" haha

did you check your alignment? or tire pressure?

Transmission mount, Transmission mount, Transmission mount, Transmission mount, Transmission mount, Transmission mount, Transmission mount.

Did I mention Transmission Mount? (or bad U-Joint)

I'm going to be swapping out my tranny mount for this very reason.

I have had this similar problem with my sa. It has that vibration at 45mph. So i guess its time for new transmission mounts. (everything else is so smooth though...??)