when i put it in 4x4 it knocks/cluncks when i give it gas. what could it be

thank you for your help. i had it checked out, and it was a bad front driveshatf. now i need to know how hard is it to change for someone that is somewhat, but not great at the mechinical side. just never replaced anything like this before. thanks again

If you are sure it is a bad front driveshaft, Good luck. Just remember both front driveshafts (or half-shafts) enter the tranfer case as well. So I don't know what kind of check was performed but good luck to you.