I can't get the 2nd video to load either
From the 1st one, I'd say you don't have too much to worry about. Perhaps a bit of stiffness from arthritis as Doncat has suggested.
Arthritis in the front legs is quite easy to see. (Picture on link below):
http://www.medirabbit.com/EN/Bone_diseases/Osteo/arthritis.htm
Advanced arthritis of the front legs is even more obvious because the feet start to curve under and you see bunion type bone spurs. Abscesses can spring up from the pressure points because the wrong part of the foot is in contact with the ground.
Back leg arthritis is more difficult to spot, but buns tend to be *wobbly*.
I know it's hard to tell from photos, but I think Nia possibly has muscle wastage rather than being *skinny*. She seems to have plenty of flesh underneath.
Amy