Hi, thank for quick comments. When trying to shift up, it's like agains wall, shifter moves a bit but inside the transmission nothing moves. It's like 2-speed system.More information is needed.
Does shifter move at all when you try to shift?
When you lift to to shift is it tight or does it move as normal but not shift?
Does shift seem to be in the sam place as it always was or has it moved?
Upshifting beyond 2nd usually involves a racheting mechanism and camming assy...! Being as your ride is 4-5 years old with low mileage, something either broke or was bent out of positioon, perhaps even the ball and ramp racheting setup, so like others have posted, drain the tranny, look for debris on the magnet...then remove the derby and primary cover, hopefully you will see something before having to dig very deep. Here is where the rubber hits the road...get that service manual for details on what to look for. Also, make sure there is no burnt odor from the lubricant as a precaution.
aint gonna see much by taking the primary cover off with a shifting problem, more common than anything else ive seen on these is the shift lever hanging up in the bush in the forward control, this wont let the selector ratchet, i would start by checking simple things first before ripping into the gear box