To answer your one question though, as far as I know, slipper tighntness would not affect shifting. On my XXL I run the slipper 1/4 turn from fully tightened and it doesn't affect the kit shifting.
Are some 2 speed transmissions contained inside the transmission? I'm just curious because all the 2 speeds I've had have had dual spurs and dual clutchbells and were all nitros. Every electric I've had has had a single speed transmission.
Welcome to the forum. I dont know about your truck specifically (Dont know which one you have specifically) but generally on a two speed transmission theres a shift point adjustment and it probably needs to be adjusted. There should be some info on how to adjust it in the users manual.