Drivetrain slop is gone, but...

[TBone]

I have a 1994 6 speed coupe. I have been chasing the "Drivetrain Slop" for a while. All the u joints were replaced. The rear hubs were replaced. I still had a slight clunk upon adding and removing throttle. The clunk was in the area at the real of the transmission. So... I decided to replace the slip yoke and tailshaft bushing in the transmission. While the driveshaft was out I had it balanced. With everything back together all the slop is gone in the drivetain. No more clunks. But....I now have a vibration that wasn't there before. You can feel it in the floor form 70 mph on up. I indexed the driveshaft and that didn't help. The slip yoke goes in nice and smooth. I'm thinking about removing the new slip yoke and putting in the old one. I can't figure out what caused the vibration. I saw the shaft being balanced and it looked perfect. Something is causing a vibration. It wasn't there before I started. Any Ideas??
Thanks

[ChuckG]

Have you inspected all your mounts? Checked their torque?