See this alot. Generally, the reason is there is still a load against the gears. And you have to really force it to get the gears to shift. The solution is generally you have to back up/go forward a little to release the tension. My stock case will do the same thing, and I just got to release the tension, and it will shift very easily.
x2
put her in gear, hold the clutch push the shifter forward to 2wd release the clutch it works for me. i bet its common for all wheelers even stock 4x4. my cousins stock did the same he played with it for two hours then finally gave up and called me, i got there spend more time talking bs then putting her back in 2wd in a flash.