I have a 2001 f150 lariat 4x4. 5.4L and a 4R70W trans.
overdrive will not engage, will just rev the engine. If I turn off the od switch, it'll run fine just staying in third gear. There's no engine light on and everything runs smooth. I was told this could be a vacuum issue. Any ideas or info would be greatly appreciated.

 

Some trans lost a retaining holding the OD piston in position thereby allowing the pressure to apply the band to leak out.
It's been a while, suggest you verify this with a dealership or trans shop.