Hi all. I have a used 2006 Grizzly 600. I just got it a few months ago and love it. But here is my concern: it will shift from 2 to 4 wheel drive immediately but when I first got it it would take about 30 seconds of riding to switch from 4 back to 2. I had the dealer change the differential oil thinking this would fix it and now it won't switch from 2 to 4 unless I stop and put it in reverse. After a few feet it shifts back to 2. What are your thoughts?