Bent Rear Axle vs Tires Out of Round

I got an old Kodiak 400 that’s sat for a long time because it wasn’t charging. I got it up and running a little while ago but I didn’t get a chance to take it for a spin until today. There’s a quite noticeable up and down motion or wobble coming from the rear end. This wasn’t there before. I have Mud-lite A/T’s for tire, they’re a softer side wall bias tire. Could they simply be out of from sitting for so long in one position? Anyone every have this happen to them?