drive line vibration

[papastoyss]

My '40 Ford coupe has developed a driveline vibration that I can't pinpoint. It has a 5.7 LS/4l60e, 9'Ford rear. The vibration starts
around 1700 rpm & continues to app. 2100 rpm mostly in overdrive which equates to 55-60 mph. It shows up only with a load on the drivetrain, not while coasting. I have replaced the trans tailhousing bushing, rear trans mount, early Ford biscuit motor mounts twice, the last time w/Bob Drake $$ mounts. Engine/trans are angled down 3 degrees, pinion is up 3 degrees. This car has been on the road since 2012 w/zero issues until this showed up beginning last season & got more noticeable as the season went on. In neutral I can rev the engine to the 1700/2100 rpm range & feel only very slight vibrations, barely noticeable. Today I pulled the drive shaft & saw no problems , so I took it to a reputable drive shaft shop to have it checked anyway & the owner seemed to think a rear tire may have a belt starting to separate. I tried downshifting from OD to Drive, the vibration is still there at slightly higher RPM but nowhere as noticeable. Whew, long post, any ideas?

[chimp koose]

Crack in flexplate ? if it does it in o/d mostly ,that is when it is under the most load . I would guess a pinion bearing but you would hear it too .

[enjenjo]

If the tires are from 2012 too, the driveline guy may be right. You have some other tires you can throw on there just to check?

[papastoyss]

If the tires are from 2012 too, the driveline guy may be right. You have some other tires you can throw on there just to check?

That's probably my next step if there is no problem w/driveshaft. This car has a Lokar  trans mounted shifter, when the vibration starts the shifter shakes like those dogs in the ASPCA fund raising ads.