Yours seems to be at 70+ mph so I would think that you might be able to check 3rd, 4th, and 5th if the RPM's are low enough in 3rd.

If the vibration is speed related then it is most likely Case 2 and you need to check to see if it does it at this speed in all the gears you safely can. If the vibration occurs at an RPM or a given RPM range then it will most likely do this in all the gears and this tells you it is most likely Case 1. If the vibration is RPM related is typically Case 01 and speed related is typically Case 02. The reason for this is that with a manual transmission in neutral the output shaft of the transmission to the wheels will turn freely of the rest of the parts in the transmission thus isolating it from those parts. This means that the problem is either from the engine damper to just before the output shaft in the tranny (Case 1) or from the output shaft of the tranny to the wheels (Case 2). Typically it is either speed related or RPM related.

I am not sure I understand what you are saying.

88 tbird turbo coupe