ok i'm wondering whats goin on here, i just rebuilt my car a few weeks ago and i converted the 355 to a 383 stroker and i threw out the junk TH350 and replaced it with a just rebuilt TH400 with a shift kit. in the transformation i had the regular mods, custom crossmember and torque arm bracket. and i had to cut the driveshaft so i think maybe this is hte problem? well when i'm going down the road it starts to vibrate at about 25 mph and only gets worse as i go faster, the shifter and such shakes so bad and hard that the entire car shakes from it. i'm wondering what could possibley cause all of this. is it my driveshaft thats not in balance or what? the guy i had do the chopping for me has done a few of them for me already and never had a problem with them, and he runs a welding shop so he knows what he's doing but do you think maybe he made a mistake on this one and its oblong or something? i'm stumped...

 

I just went through the Driveshaft issue. First I found that the Slip Yoke was tight, meaning it didn't flop around like a noodle. It needs to move very freely. After I had that fixed, there was still some vibrations but not as bad. I had it balanced, and it drives very smooth. I've had it up to 130 and it feels good.

The reason I had the driveshaft issues is, I just did a rearend swap. I put my old 10 bolt back in (totally rebuilt) and the original
driveshaft. I guess over the years of moving, I dropped it or hit it on something.