I can walk you through the troubleshooting, but it still required a mechanic. I have one question though, "What's in it for me" as I do this? What I can present here required hours of study and a lot of hard work to learn, even though it may be able to be read in two minutes it took much longer than even that to write out.
Start by pulling trouble codes from the transmission control module. My first expectation is this should be electronically controlled. Your transmission is likely in failsafe mode and I suspect it should be starting out in second gear and staying there as opposed to first gear. Second gear is the one that the transmission defaults to when a problem occurs, the module shuts down the power relay and that takes all of the power away from the solenoids. The really tough part is the transmisison control module can make that decision in about one tenth of a second, which gives us very little time to measure as to what the computer sees that is making it go into limp mode.
Start with the codes if any, then I can devise a way for you to test.