I'm attempting to simulate a GPS message where know variables are starting location, ending location, speed (m/s), and a bearing (0 - 359 degrees). How can I calculate the ECEF X, Y, Z velocities with this information when I am not tracking any satellites.
Assume that the motion is on the surface of the earth.
You have too much information. Given Position 1 and position 2 and the knowledge that you are following a great circle, you can calculate the bearing and distance using the formulas given in the Aviation Formulary
You were given you the speed in meters/second. You can calculate the XYZ for the two points. Use the delta x, delta y, and delta z, and the time to move from point a to point b to calculate the velocity components.