I have a 2007 3500 5.9 (disregard the title of the thread, my old truck was an 06) and my abs and brake light are on.

Put it in 4hi on the fly, and the light just blinks.

Stop the truck in neutral and put it into 4low, everything goes normal and the truck engages into low.

Move the switch to 4hi and the truck engaged.

So right now the 4hi will not engage or disengage unless I put it into 4low first.

Also when driving, the speedo will drop to 0 for 10 seconds or so, then go back to normal.

Don't have a code reader either......but thought someone here may have had this happen to them.

 

Mine acts kind of like that only when it is real cold like -10 degrees F or lower

I have a 6sp so I don't know if that makes engaging it different.

 

Hmmm.....don't know if the cold has much to do with it. I only have had to use it since it snowed. Mine is an auto.

 

No one else huh?

 

Maby it has something to do with a wheel speed sensor being out since ur abs light is on and it wont shift because it doesnt have a reading on speed but im really not sure if that matters or not because i have no idea what parameters it requires to make it shift but its a avenue to check out good luck