07 fourtrax stalls when in gear but not nuetural

hey everyone. im new to the site and have a question that i hope someone can help me out with. i have an 07 honda rancher 420 cc engine. ive done the oil changes and oil filter but never the fuel filter or anything else. the bike has been running fantastic until about a week ago. heres what it does now. it starts like a charm, idels great and you can rev it and everything is perfect. i forgot to mention it's one of the semi automatic trannies also so no clutch but manual shifting. well once u put it in gear and start going it stalls right out. i can kick it into nuetural and it will stay running and running normal at that. i'm just not sure why it is stalling on me when it's in gear but runs great in nuetural. i just don't wanna go buying parts for it that i don't need right now so if anyone has any ideas or has had this problem befor could you please let me know. thanks everyone i really appreciate it.