I have been scouring the Technical Service Manual for the TX6A and what I have been able to come up with is that it looks like the Locking Clutch Assembly is controlled by several sensors including the Linear Line Pressure Sensor as well as Turbine Shaft Input Sensor, Vehicle Speed Sensor and ATF Temp Sensor.
It seems that in the event that the locking clutch mechanism disengages for any reason more than twice in a drive cycle, a P2757 code will be stored. One of the failure thresholds is if pressure to the Lock up Control Solenoid Instruction Valve reaches 513 kPa or more.
My first guess is that there is an threshold algorithm that is a combination of line pressure values, impeller speed, vehicle speed, and transmission gear that is being exceeded when RPMs exceed +/- 7500RPMs which would result in the locking clutch disengaging suddenly. I'm willing to bet that it would do this in 4th, 5th and 6th as well but please for the sake of public safety, don't try it on public roads!
My second guess (and more likely) is that the transmission control module (TCM) is trying to get the transmission to shift into 4th based on impeller speed and engine RPM but the manual shift control circuit is not allowing it to happen.
Of course, this is purely academic and I would STRONGLY suggest you do NOT allow the clutch assembly to disengage at that RPM due to the damage that it can cause to the clutches, the heat that it will generate and dramatic fluid pressure spikes that will result.
Scott