The T90 does have a common issue with popping out of second gear. It is related to forward and aft movement of the main output shaft. It can be fixed in the vehicle without removing the transmission. At one time, there was a shim kit available specifically for this problem. It contained various thickness shims which were horse shoe shaped. You would pick the thickness of the shim which eliminated most of the shaft movment, slip it into position, and then bend i the ends together to keep it in place. I have not seen this kit being sold for some time now but you can make your own by using tin or sheet metal of a thickness that works and cutting it to the correct size.
1) Remove the plate off the back of the transfere case to expose the large gear which is bolted to the transmission output shaft.
2) remove the bolt attaching the gear to the shaft ( you don't need to remove the gear, just loosen it somewhat so it does not restrict shaft movement.
3)remove the floor plate and then the shift tower from the top of the transmission
4) Push the output shaft ( the one attached to the transfer case gear) forward and look at the movement between the back end of the output shaft and the rear of the transmission case. (it doesnt take much movement or looseess here to cause second gear popping out problems)
5) Locate a piece of metal which fill this space and mostly eliminates the fore/aft movement. Cut it in a horshoe shape and slip it between the back of the output shaft and the transmission case. bend the horshoe tabs together to keep it in place.
6) reinstall the transfere case gear, nut, cover as well as the transmission tower and floor plate.