I finished my Tr3650 to T56 swap and took it for a test drive. It drives fine until you get above ~40 mph, and then you start feeling a slight shudder, which becomes more noticeable on deceleration. It's to where you know there is definitely something wrong.

To help diagnose, here's what I have done (and therefore may have screwed up):

1) Swapped to a rebult transmission. (filled it up until oil started coming out of the fill hole).

2) Swapped to a shorter driveshaft (which I understand has been properly balanced....?).

3) Changed my rear end fluid (filled it up until oil started coming out of the fill hole).

4) Changed my motor mounts.

I should also point out that last week I had the car together and but the tranny had to come back off due to a plug the builder forgot to install. Regardless, I took it for a spin to to check the operation of the tranny and the car seemed to drive fine. I don't remember a shudder then. Soon after, I remove the tranny again and reinstalled.

Previously, the driveshaft I got for this swap was on the car for roughly 5 miles before I removed the tranny again to install the plug. When I re-installed it, I am pretty sure I did install it in the exact same way it came off (same holes and clocking on the pinion flange), but then again I assumed that this wouldn't matter due to it's being on there for only 5 miles.

One more thing. To get the driveshaft off, I had to place a block of wood against the u-joint and smack it unil the shaft came free. I assume this is safe to do and I didn't mess up the alignment of the pinion??

Anyhow, that's everything I did. Anyone see anything that I may have done incorrectly to cause a shudder at 40+ mph?

Does the driveshaft simply need to be balanced? I think my car is the first one it's been on since it was shortened and balanced so that doesn't make sense!

Thanks for reading all of this. I appreciate any help!!