60mph in 6th gear vibration

when im going 60 in 5th im turning aroun 2,100rpms and get "no" vibration. I say "no" because I have solid mounts engine mounts and a poly trans mount, so there is always some vibration :P. Anyways, when I shift into 6th the revs drop to around 1,500 and this vibration starts. It is not the DS problem, because when went to test that the other day on my local empty highway I went to 80 in 5th and then shifted into 6th, and there was no vibration. My poly trans mount seems to be intact, and all bolts seem to be tightened, so I assume it is timing related.

Here is a copy of my current tunercat tune:
http://members.aol.com/atomicm0nk/97ss.bin

I can take a datamaster log of whatever you guys could use to help figure it out.
Thanks

This will happen in most m6 cars because ur lugging the motor at 60 but with your 383 you should be making more torque down low so...

I know when I first purchased my car 6 months ago with everything done to it I could go into 6th at 55-60 no problem. But that was before I changed the injectors to 36#ers, replaced the clutch, tuned it many different times, and many other things. Thats why I think it is timing related. I cant seem to find the origional tune right now, otherwise I would just simply compare the file to the one I have now. I know I saved it somewhere but between my computer, my parents, and my laptop I cant seem to find it :/ Tomorrow I will try to tighten the bolt that connects my poly trans mount to my crossmember, it was tight when I put it all together, but it might have loosened. Would that cause a vibration at 60, but not at 80? Any other ideas?