The height of the hitch. Underslung will raise the center of gravity.
Yep if that is the case the first thing would be to raise the hitch height with a packer.
There is a thread here discussing it and Bushman has a picture of his packer on there -
http://expandasdownunder.com/threads/50mm-hitch-packer-where-to-get-one.332/page-2#post-2955