I got the 71 240z with a five speed upgrade...when i hit about 65mph in 5th gear iam getting this vibration thats deep and loud...so far we did,,new u joint and rebalanced the driveshaft...new halfshaft u joints..wheel bearings were done last year by PO...Has anyone seen this before? whats next is the rear...Any ideas/HELP....thank you

Have you checked to make sure all mounting fasteners are tight? Make sure that all driveshaft, diff mount and trans mount bolts are torqued to spec.

Could be driveshaft angles. Hybridz.org has some good threads on how to measure the angles with modified laser pointers (usually done on an engine swap). I know the argument is going to be that the 5 speed is the same size and so the angles should be the same, but I think it's work checking, especially if you've tried everything else. On the early Z you can change the angles pretty easily by shimming under the trans crossmember. I think that changed mid 71...

I know it sounds simple, but have you checked your exhaust piping to make sure it isn't contacting your diff.? I have 2 1/2" piping and it comes real close to the diff. I had to move it a bit because of a deep vibration when accelerating hard. Just my 2 cents.

Cheers, Mike

I got the 71 240z with a five speed upgrade...when i hit about 65mph in 5th gear iam getting this vibration thats deep and loud...so far we did,,new u joint and rebalanced the driveshaft...new halfshaft u joints..wheel bearings were done last year by PO...Has anyone seen this before? whats next is the rear...Any ideas/HELP....thank you

Deep and loud implies something with a lot of mass and/or relatively low frequency. Tires and wheels come to mind. What happens if you shift in to 4th at 65, besides the higher RPM? Does the vibration change? Push the clutch in and see if it changes too, this will separate the engine and flywheel from the wheels and drive shafts. Might tell you something.

I had the same thing happen to me when my diff went out. It started while driving 75-80mph in CA traffic. Then progressively started getting worse until it was happening at 40mph. Finally heard a loud clank in the diff and it did it at anything fast than 30. Haven't driven it since as I restore it. Elaborating on Zed Head's comments above, here are the same questions and why they apply:

1) Does it happen in 4th gear at 65mph? If yes, then you're probably looking at drive train linkage behind the tranny (diff, driveshaft, u-joints, etc).

2) If no, you need to rule out the transmission. You can do this by simply releasing the clutch at 65 and shifting into neutral. If it's still there, you're probably still looking at linkage in back of the transmission.

3) If yes, then you can start thinking about the transmission, but it's kinda doubtful.

My bet is on the diff or wheels/brakes. Just my opinion. Hope that helps!