You can buy a small accurate clinometer at a boating supply store for $10 or so. But clinometers made for boats don't look so hot in vehicles and you'll have to be creative mounting it. The ones for sold for 4x4s showing both lateral angle and incline are usually so small and graduated so coarsely that they don't tell you anything useful. Make sure its dampened or it'll bounce around so much it'll be useless.
IIRC, few months back someone posted a pretty slick one here on the H3 forum that they designed and installed , but I don't remember who!
And I assume you know 40% is a little under 22 degrees.
This outfit makes them for the construction equipment:
http://www.tiltmeter.com/page9.html
Here's one for the truly paranoid:
http://www.tiltmeter.com/page25.html