Ok ladies and gents..I have an 06 scab 2wd. earlier this month while on a trip to MO my ABS light came on. Being the kinda guy I am, i had my scanner with me. I received a code for "rear speed sensor malfunction". cleared the code light went out no big deal....once we got to our destination shut the truck off, restarted a few hours later and the light ws back on. same code got a new sensor replaced and the light and code were still there.
Did a little diag. checked all wires etc etc. found nothing. I figured that i would mess with it when we got back home. while driving back home the light sent off by itself...the truck was shut off and started 3 times and no light...HOORAY!!! i thought..next morning light was on again..
research and found some info that the ABS module could be bad..replaced that to no avail.
so im kinda stumped and hoping that my F150 brethren could help with ideas or experience..
thanks and sorry for long winded post!

 

Turn key to 2nd position, open hood disconnect neg. on battery. Wait 5 mins. Key still on 2nd position connect battery. Should work. If light comes back it's a bad abs module. Good luck!