I have been having the same problem and thanks to this forum, I called my dealership and found that there is a Technical Service Bulletin (TSB) for this issue. I describe mine as feeling like I am driving over rumble strips. The dealer confirmed that the TSB will fix the problem - it may not just be a reflash, you likely will have to have your torque converter replaced. Mine is on back order as of yesterday. Sorry, I don't know the TSB number but I can tell you that it was just revised again on 2/10/07. Definitely call your dealership. I also had a vibration at 40-45 and found out that my Nitro had a manufacturing defect in the front differential - it was something the Dana acknowledged that they had a problem on the assembly line. If you want more information on that, I can get it - I purchased my Nitro in December so it may affect those built around the same time as mine (I can get that info too, if interested). Good Luck!!