Heycuzzz

Hello, having an issue with the drivetrain on my buddies atv. We've replaced the primary, and secondary clutches. It starts fine, runs good, put it in gear, and nothing happens. Won't move forward, or backwards. When we jack up the rear end the tires spin, go thru all the gears, but when there is any resistance (sitting on the ground), it won't move........is it a drive shaft issue, or a rear end issue. Any help with this issue, would be greatly appreciated !!!!!