Suzuki grand vitara 1.9ddis, when changing from 2nd gear into 3rd a slightly notchy/ catching og gears is experienced.If the clutch is pressed and held down for a few seconds the change up is pretty good.I do not feel the same notchyness when changing dowm from 4th.

This could be more than one reason. Could you please comment below describing exactly what happens when the pedal gets 'stuck' down?
Does the pedal feel correct as you push it down? Does it change gear normally even when the pedal does get stuck?
If the clutch pedal feels wrong when you use it, the clutch fluid may be low needing to be topped up and the system might need bleeding. Is the pedal slack when you press it or pull it free? This could be a faulty return spring on the pedal.

The fact that you have to switch off to get it into gear suggests a cylinder problem.

Check that the cylinder isn't leaking (and there's fluid in the reservoir..).

Clutch cylinders aren't overly expensive:
617results for vitara clutch cylinder

... and if you hunt around you may even find an overhaul kit which is even cheaper.

what is stiff (add a noun here)
blush.....
clutch pedal stiff
shifter lever stiff.
grinds , a bad clutch will make the lever seem stiff or will grind, if forced.
gears balk? same as stiff.
false neutrals. (slips , gear not engaging)

this part time 4WD? option>?
are you on ICE, snow, dirt or pavement.
part time is not for pavement free of ice, you know that?

only AWD works on non iced pavement.
RTM

sounds like your clutch worn and pressure plate needs renewing

vss is bad

If it's hard to get into gear you should check the clutch is disengaging properly.
Before changing the clutch plate, check that the clutch throw-out is adjusted correctly and the actuating hydraulics/levers have the correct amount of travel.

Release bearing problem.Change the clutch Assembly

Try hammering the front right hub with a wood or a rubber hammer while slowly reversing the vehicle, and keep on unlocking the hub while doing this.
spray it with lubrication first
yes, you need to change the gear oil in your gear box
Have your clutch re adjusted, it may be worn out , that's why you are having a hard time changing gears or have it bleed to keep the air out if you are using hydraulic.
If you dont hear any noise in your gear box while driving, its nothing serious, worst thing could be replacing your clutch lining and pressure plate.

You can try using synthetic gear lube, I have found it reduces shift effort by at least half when it is cold.