Do you mean something like an inclinometer? I know the old Toyota 4-Runners used to have them built in to the dash. In fact, I think the new FJ Cruisers have them. Good for serious off-roading. I know I've seen aftermarket versions around, but I don't know about digital. Here's a link I found with a quick search. I think it's for an ATV/UTV, so I don't know how it would mount in your truck... might look goofey. Well, now you've got me interested. I'll let you know if I find something better.
Trace
http://www.cabelas.com/prod-1/0046192523067a.shtml