A 1996 with 95K miles... When I start from a complete stop, the RPM's increase slightly and truck accelerates. Very soon after I take off I get a mild "thud" from the drive train (I think from the drive train), like a very subtle gear shift but it is not a gear shift. It feels like the drive train is slow to engage and then fully engages when the "thud" occurs. It is fairly subtle and only happens about half the time. Any ideas? Ive only had the truck for about 4 months. It has always done this.