no power windows, abs light on, no speedometer, tach works

Yes, there is a BCM which is called a CTM on the trucks, The only item the CTM controls on your list is the ABS Lamp. Power windows and Speedometer are not an output of the CTM. Since your speedometer does not work and you ABS lamp is on. I would check the vehicle speed sensor (VSS). The code 950 means the cluster is not receiving an ABS lamp-on message from the Controller Antilock Brake (CAB). Check the CCD data bus connections at the CAB.. Check the CAB fuse. The code 999 means An error has been discovered.