Vibration after transmission and rear drive shaft repair

Just had my automatic transmission rebuilt and the rear drive shaft Ujoint replaced (it had a lot of slop). The car has 177,000 miles on it.
Now the car has a vibration at between 40 to 50mph. Took it back and they said they took the entire drive shaft out (both halves) and had it re-balanced. They tell me that once put back together, that the vibrations were better but not gone. I asked it they checked the center carrier bearing and they said it was fine. Now they tell me they need to remove the rear axles to inspect to see if that is the cause of the vibrations.
Has anyone ever experienced having the rear axles start causing a vibration after a transmission and rear shaft repair?
This doesn’t sound logical to me.

And what kind of car is this again?

1990 300zx 2+2 NA

Make sure your motor mounts, trans mount and any diff mounts are in good shape

Bad mounts will result in incorrect driveline angles, which can contribute to vibrations

Just throwing it out there . . .

Have you tried running it up to speed, shifting into neutral, and then coasting to see if the vibration is still present?

It sounds like a bad U-joint to me. I had a similar experience when I had a driveshaft U-joint replaced on a RWD Toyota Previa. To save money the dealer put in an aftermarket U-joint and, after three visits back to the dealer for the EXACT same condition you are describing, they put in a Toyota OEM unit. The vibration disappeared.

ok4450…yes I tried that and the vibration was still there, even though I was not giving it any gas.
db4690…Could it still be bad mounts when I get the vibration in neutral?

Vibration in neutral supports my bad U-joint theory.

Yea…I agree…I had them replace ALL the u-joints and re-balance… and now the ride is much better, however, there still is a slight noticeable vibration throughout the speed range starting at 35 and up. Only thing I can figure now is somehow they didn’t put the two shafts back in proper phase with each other…even though they say they did. Not sure what to do next. I hesitate to buy a one piece shaft but may have two.

Are they using OEM U-joints? I don’t use OEM for all applications but two places I prefer OEM are ignition and drivetrain parts. Good luck and I hope you get rid of your vibration.

Hard to know…they say they did, but how can I tell? I may have to take it to the Nissan Dealership to check for sure.
Thanks for the help and advice.