When my friend is going at least 70 MPH the overdrive doesn't work.
And sometimes the overdrive doesn't work when he's going 40 MPH!
What could I do to fix the problem.
And I would like to avoid going to a repair shop.
Overdrive is misunderstood.
The traditional definition is:
a gear that drives the rear rotations to be lower than that of the crankshaft.
That means you truck shouldn't go into 'overdrive' at 40mph. No car does this. At a higher speed, in order to conserve fuel, a car may go into overdrive.
Knowing if your vehicle is actually going into overdrive, or not, is not a no brainer. Making a claim of, 'it doesn't go into overdrive' doesn't mean it isn't, it only means you believe it isn't. Two different things.
What are the facts regarding your vehicle "not going into overdrive". If you explain that claim then someone here can help you to resolve why it isn't functioning properly.