Thoughts on vibration

Yeah my guess was that you went for 119 to 124 mph and the drive line was having a frequency problem at that point.
Run threw it at the top of the track, and then back threw it as you decelerated. Where before you never got there in mph(driveline rpm) to feel it..........

It not eng vibration, or you would feel it at every gear shift............is your shift RPM the same as your threw the trap rpm???
I guess if the vibration is minor and isn't getting any worse.

Is it possible that the transmission tail shaft bushing, is getting a little bit loose? or slip yoke has some wear on it. Drive shaft guy should have noticed that when rebalancing your shaft.