vehicle speed signal F250 diesel

Factory service manual perhaps? I haven't looked at a Ford manual, but the two Chev. service manual sets I've gotten for my truck have had this information so I would expect the Ford manuals would as well. If you have good connections at a Ford dealer you might be able to look at their manual for the info, otherwise the proper manual set is usually around $100.

Another option would be to simply probe the harness to the cruise control unit, presuming it's a self contained unit like I see on the GM stuff. Only a small number of lines going to it, and the speed reference signal should be easy to isolate if you've got a scope as every other signal to the unit should be pretty static i.e. power, ground, brake switch, clutch switch, decel, accel, etc.

Whatever you do, be sure to properly buffer and protect the signal when you tap it so you don't risk damage or malfunction to the ECM or other units that use the signal. There have been a few good articles on signal conditioning in an auto environment in Circuit Cellar in recent years.

Pete C.