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[toyomoho]

I am not an autotrans expert, but.

You think the torque converter is locking up (or locked). This puts a load on the engine when the trans is shifted into gear killing it. As I understand the autotrans, the torque converter is in non-lock up mode until a certain speed (MPH) is achieved. Provided of course the 0/D switch is on and the coolant temp is up. The O/D solenoid controls this operation. This O/D setup appears to have its own MPH sensor just for the O/D. This is separate from the MPH sensor for the gage on the console.

When the engine is at idle, does it sound under a little more load then normal. As if the converter is engaged. Perhaps for some reason, when you shift into gear, the converter engages.

Guessing either the O/D solenoid is bad and holding the torque converter engaged or the converter has a problem. The converter has a one-way clutch that if bad can cause it not to disengage. Perhaps this is why the dealer replaced the unit. Not sure a replacement would go bad so soon but stranger things have happened. If so, I would think they need to replace it on their dime.

Or maybe the trans is attempting to shift into two gears at once. This might be an ECU, gear lever position switch (tells the computer which gear you have selected manually), or one of the other solenoid is acting up. This switch is external and can be easily tested. The autotrans has 4 or 5 electric solenoids that control shifting and O/D operation.

If this car is 4WD, maybe there is something screwed up with the 4WD engagement system.

There is also a chance a trouble code is being stored for readout. The solenoids are controlled by the ECU that monitors their on/off performance.

I can say a competent dealer should be able to figure this out. The Toyota shop manual should provide everything needed to diagnose the problem. These manuals are very detailed! Assuming the replacement converter was not defective (these things can happen), the dealer needs to try a little harder to find the problem.

And don't rule out wiring issues, as in bad or loose wires. But all of this should be covered in the shop manual.