Now this is a recurring problem, but I haven't really taken it seriously before

Shifting from 2wd to 4H is nice and smooth, but after driving in 4H for a bit, shifting back to 2WD is almost impossible. Have to really wrench the stick into 2WD (and I don't mean a bit of force, like smashing it!!

Also sort of sticks inbetween 2 and 4H, and then if you drive it in/bw in pops back into 2WD on its own, again with a bang.
Anyone know what this is? Do I need a rebuild?

Also - going lower transfer gears (eg calmini kit), does this lower the top end gearing of the car (ie, and I going to be doing 5000RPM at 90Kph)?

Ado