1)
Which mountains danced?
1.
Radak: Sinai danced - "va'Yecherad Kol ha'Har Me'od" (Shemos 19:18). Perhaps also mountains near Sinai trembled and quaked.
2.
Malbim: This refers to Sinai and Hor ha'Har, and the miracles done there 1 .
1
Granted, Hor ha'Har [and Har Nevo] were the only other mountains that the cloud did not flatten, and Aharon's death was miraculous. However, where do we find that it danced? (PF)
2)
Why does it mention "Harim" and "Geva'os"?
1.
Radak: Geva'os are similar, just they are not as tall and pointy at the peak as mountains are.
2.
Malbim: Geva'os are smaller; they danced like young flock.
