Perhaps the least changed feature of a truck in mtm2 is the axle bar texture. This is the part that extends from the front to rear axles with a pivot point in the middle. There is one for each side of the truck.

Just like shocks, this is a 64x64 raw file that is stretched to fit between the axles. Again, this is rarely changed, but if you want the axle bars to match the undercarriage of your truck then you may freely change the texture using Tracked2. When you're ready, press "8" on the keyboard. You will be prompted for the texture name. Type it and press enter. If the raw and act file is in the art folder, the new axle bars will appear on your truck.

And, of course, the information in the bottom left corner of the screen will be updated to show your new shocks.