Okay, so you’re not stranded, which is good.
First thought is that the clutch assembly is toast. If the clutch becomes oil contaminated or completely worn out or jammed disengaged, it will do nothing but slip and you will be stuck. Hydraulic piston assembly failures are one common cause that is easy to repair. Have the shop check this if they haven’t already. It it is stuck extended, the clutch will never engage.
Second thought is that the transfer case is damaged. To test it, you would need to separate the transfer case from the transmission and turn the input shaft and see if it turns normally.
It could be the transmission, but usually that fails in other manners, like grinding or complete destruction.
Has the shop separated the transmission from the engine yet?