On a warm day in Florida, when I start my Diesel, the car idles low enough that the Tach does not come off of Zero and the Antilock light stays on. [Like it is dieseling].

I can drive the car and after a few minutes the tach jumps to a normal reading and the light goes off, so I am sure the antilock light is on since not enough vacuum is coming from the engine.

Has this been seen before or could this be a sensor going bad?

I have over 200K miles on this car and it is still running strong. And this is not every time.