I have an 80 that my son flipped! once back on level ground the rear axle is seized up!! any suggestions on why what area to look at?

 

First thing I'd check is to see if the engine cases broke (front sprocket area and where the rear end bolts to the cases). Take the front sprocket off and see if the rear sprocket moves and the shaft that holds the sprocket turns.
If everything is fine there, it would have to be in the rear end. Back the brake off. If that don't help, guess you'll have to take the brake cover off to inspect, If alls good in there, turn the axel, maybe a axel brg went bad.