Re: Stuck in 4 Low & Service 4wd System
Ideally, you want to be rolling slowly forward to engage/disengage 4-Low, as this helps to synchronize the gears (as the OP was doing).
It is possible to engage and disengage while stationary, although this is usually a little bit louder and you may have to attempt it a couple of times before it takes.
If it threw the Service 4WD message, I would take it to be scanned and see if that provides any insight into why you couldn't disengage.
I would be hesitant to put it back into 4WD-Low without knowing what caused it, as you say, you don't want it to get stuck and not able to go back into high.