There're two bars that run parallel to your frame under the truck. At the back there are two bolts. Just jack up the front end and tighten the bolts, and it will level the front with the back. If you do it though, you should get an alignment afterwards.
This diagram shows them well http://www.4x4abc.com/images/tech/torsions.jpeg
__________________ 2001 Ford F150 XLT 4x4 Offroad 5.4L V8