Mine has done this also for quite a while (5+ years) but only when it's cold outside and the truck's been sitting in the cold. When it's above, say, 50 degrees outside it never happens. Based on this, I've always figured it was due to lubrication.

It's on my long-term to-do list but not that important to me since my speedometer is off anyway because of non-stock gearing and tires. Presently, it reads reads fast (i.e., shows 70 MPH when I'm actually doing less) so I tend to just go with the flow of traffic or guess-timate based on RPM and what gear I'm in.