I'm getting some horrible vibration from the drivetrain/tranny at around 100-110mph. What would cause this? Would an incorrectly installed driveshaft cause this? Or would the vibes occur much earlier from that?

 

does it do it when the car is in neutral and you rev it up?

 

No. Only does it when I get up to a high speed. If I row through the gears at WOT.. the vibe starts growing where at 100-110 it gets bad (in 5th).

 

Stock driveshaft will cause this. Try turning it 180*, if that doesnt help, then go with LS1 or 1LE alum shaft, or a custom steel shaft. This is a common problem.

 

That problem is so common--unbalanced driveshaft--that mine was replaced with a 1LE driveshaft under warranty. There is a Technical Service Bulletin out on this.

 

I guess I should have mentioned this: I already have an LS1 driveshaft installed. I guess I'll try rotating it 180 degrees.

 

Maybe you've got a bad tire? Sometimes tires will cause a vibration at various speeds, and often you can "drive through" that critical speed. If the D.S. doesn't solve it, maybe get your tires checked for balance, etc..