Sasha wont shift into overdrive after being swamped

I put my truck into a pretty deep creek and have been fixing it ever since. ive rebuilt the engine and hooked everything back up but im at another problem, no only do you have to use the little button on the shifter to get the truck out of park but driving down the road she will go all through her gears just fine but will not shift into the final gear. ive been flushing the tranny fluid on it and this is the third flush and its getting expensive. I already have a similar thread if any of this seems familiar.
So I guess what I need to know is does anyone have any ideas on what to do to fix it? Its been like 3 years since ive tanked it so im really itchin to get her goin again.
any knowledge or consensus is greatly appreciated!

 

Do your brake lights work? (i swear im going somewhere with this)

And are you sure its overdrive and not just the lockup clutch not engaging in 5th gear.

Reason i ask is if you are having to push the button to get it out of park every time, the trans is not seeing proper brake switch signal. And when the trans sees a brake signal it does not allow the trans to lockup the lockup clutch.

So id start looking at that first