I have a 1991 FJ80 and it has a very annoying habit. When im getting up to hi-way speeds and hit 50mph the converter locks up. This is fine but when i hit a hill and start to accelerate the truck goes into 3rd rather than stay in 4th and un-lock the converter. This is also annoying when accelerating hard and it goes into 4th then 1 second later drop into lock-up. Is the converter lock-up electronic or is it mechanical inside the tranny? If it is electronic I want to put a switch on it so I can control when it goes in and out kind of like an overdrive button. If anyone has done this or has any info please let me know, thanks