tko 600 2nd gear problem, please help!

[garry viohl]

428 fe with tko 600 .64 5th
4000 miles on tranny

bad vibration (in the gear shifter and seat) and only in 2nd gear from 4000 to 6000.
no vibration in 1st or 3rd from 4000 to 6000.
drained oil no metal parts, had royal purple snycromax
shifts fine, no problems.

Ran fine until I ran 4 laps at the track for the first time, but was not pushing it hard.

Has anyone had the same problem? Should spend the money to fix this tranny or go toploader?

Thanks for your help.