Tractionless

My Godzilla shaft was out of balance and once brought back in the vibration went away. It would also happen beginning at 50+ mph like you're experiencing.

Have you tried indexing the driveshaft to each of the 4 available positions on the rear end flange? This is even mentioned in the factory service manual for the OE composite shaft.

I used a poly mount on the OE Z trans and the vibration was insane so i went back to stock. Try modding your OE rubber trans bracket to fit and see if the issue goes away. If it does you can rule out the driveshaft.

If you still have the vibe. the next place to check (unfortunately) is the flex plate. Being I'm not boosting my Z I kept my good condition OE in place during my VR swap.

BTW, don't expect any help from Godzilla with the shaft. They threw every excuse at me regarding the issue being something else, dimensions that never changed like shaft angle (made up with u-joints), distance between trans. and rear end (made up by their own shaft length), rear end angle (didn't budge during trans. swap lol), etc.. When I sent them pictorial proof and receipts proving the shaft was out of balance they completely ghosted. I guess not refunding the $75 i spent on balancing, not including 2 one hour round trips to the driveshaft shop, 2 hours indexing the shaft to the 4 available positions on the rear end flange is more important to them than keeping a customer that will recommend their products.

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Toys Not Tots