Intro with a driveline vibration

[AlexW]

Hi all, been reading around on here lately to learn some tips and tricks on working on my 1985 GMC 3500 dually. It's the High Sierra 3+3 package, 2WD.

The transmission went out on it, and I replaced it. I discovered a rotted out carrier bearing and a bad u-joint on the front half of the driveshaft. I had both of those pieces replaced, and put the driveshaft back together the same way that it came off. Though, it looks like it can only go back together one way...

I'm having a driveline vibration problem. I don't feel anything at all until it hits 40MPH, and then it will smooth out some by 55MPH, but it is still present. I haven't driven it faster than that since the swap. Do I need to have the driveshaft re-balanced? I'm a bit confused about what could be causing this. Prior to the transmission replacement, I had zero vibration issues with the truck at all.

On a side note, possibly related, I removed the transmission mount to get the transmission out and have been having a beast of a time getting the bolt holes to line up. Is this mount only designed to be installed one way, and it could be that I have it installed backwards?


Regards,
Alex