truck goes in 4wheel high back to 2 wheel just fine but will not go into 4wheel low light blinks but does not engage.I think it may be a problem electronic sensor if so where would it be located.

 

At the risk of stating the obvious, do you have the truck in neutral, perhaps rolling very slightly?

 

i have had it rolling and stoped in neutral still no luck

 

I am going to assume you are verifying it by driving and not just the light.

It could be the actuator is binding or there is an issue internally with the t-case.

 

Im sure its not the light it just will not go into 4 low

 

Had the same problem on a 08 2500 last summer. Had found the toyo mt that i had on the truck had wore different due to air bags in the back. The problem started after rotating the tires. I replaced the tires and the problem went away.