That is not necessarily true. A Suzuki Samurai weights less than a Escape and is a great off road vehicle completely stock. However a Samurai has a true frame unlike most CUV's like the Escape, Liberty, etc. which are unibody construction. Unibodies can be beefed up however. Look at the Jeep Cherokees which are also unibody and are very capable off road too.

gregorymp - hopefully others can help you as I have not looked into modding my 2007 Mariner yet. What I will tell you is try to keep it as low as possible as the more you raise the COG the worse the vehicle handles in off camber situations. Even in fast fire road switchbacks the lower COG will help a lot. I view the Escapes and Mariners sort of the way I view my former Subaru Impreza RS 2.5 that I had prepped for rally racing.