not rbv but f150 1999 abs light

[alaskan155]

i have a 99 f150 4x4,
every once in a while if the rear tires loose traction(sometime not) the abs light will come on and stay on until i restart the truck then it will go out

is the the rear vss sensor or something else?

the pad them self look like that are at 40-45%

also in the morning the brakes are real touchy the first mile, then they are fine
and when the abs light comes on they are also real touchy(dosen't take much to lock them)

what could be the cause of either of these

its a 1999 f150 5.4 auto 4x4 74k miles
has 4wheel abs, and front disc/rear drums(i got jipped being an early 99, last 99 had rear disc :()

thanks