Welcome to the forumz to each of you. You should stop by the Newbie section and introduce yourselves if you haven't already.
The jacked up tach is a notorious problem on 2006 trucks. My 2006 4.7 did it too. I did a lot of reading and came to the same general consensus about what causes the problem....most likely the stepper motor. There is a company that I found online....cluster doctor or something like that....that will fix it and give you a couple year warranty. The price was around $180 and I think it included shipping both ways. I intended to pull mine and take it to a local electronics repair shop to see what they could do but I ended up trading it before I got around to it.
**EDIT** I searched and found the site I was thinking of. $179 with free shipping both ways and a two year warranty. They specifically list the problem that the 2006 trucks have.