03 F150 4.6 Auto 4x4 Ext/C S/Box, minor slip on take off (no thunk sound). I am 2nd owner, and know that 2nd gear has always slipped or been useless. Slip has been happening for a few months now.

This slip reminds me a bit of similar slip I had following my 98 clutch replace, but that was a manual tranny (and also the recent rear end failure, rebuild question in a different thread).

Appreciate comments before it gets worse.

 

What's the mileage and service history of that transmission?

 

What is it you expect to get from this thread but its time to rebuild the transmission?
There are clutches in it that wear out.
There are Servos that could be in trouble.
Valve body could have an issue.
There is no magic from the outside to fix this..
Good luck.

 

I agree with Bluegrass. At this point with the history you describe the next thing is to take it to a transmission shop. One you can trust if one exists. Chances are it needs a rebuild and has for some time. If it did this since new then someone dropped the ball by not getting it fixed under warranty.