Driveline vibration problem
I've had this vibration since assembling this car. Starts at about 40 and I can hear it more when I let off the throttle at that speed. By 60-70 it is really strong, like a repeating harmonic. It chills out a little by 80mph.
I bought this driveshaft new and I shortened the explorer 8.8 that is in there now so it could be either one. I also have custom trans mount and RMS lowered motor mounts, so my angles might be wrong too.
I already had the tires rebalanced (but it doesn't feel like any of the numerous tire imbalances i've had). I then measured my driveline angles and got motor down 2 degrees at the trans output side, driveshaft is front-up one degree, and the pinion is 1-2 up. Maybe too straight?
Any ideas before I take it in to the shop? Is there a way to tell if it's a janky axle or if the driveshaft is just out of balance?
1966 Mustang 302ci 4r70w, csrp front discs, 8.8 rear, EPAS, Fitech for fuel