10 team PPR League. Which RB do I drop and which one do I add? Or stay put?

Ajayi seems to be settling into some kind of lead role, which makes him better than any RB you mentioned here (although I don't know which D. Washington you have). At this point, I'd probably dump Riddick. The Detroit backfield probably should be avoided completely unless there's a drastic change.

I'd also try to pick up McKinnon. The Redskins proved yesterday how easy it is to beat the Eagles using a strong running game, and I imagine this will be used this week. I'd dump Washington, I guess, especially if it's the Detroit Washington.

This doesn't solve your problem, though. This might be the scariest RB corps I've ever seen in a 10 team league! If you're really strong elsewhere, you may need to put together some kind of combo deal and see if you can snag a guy who isn't sharing carries.

Pick up Ajai and drop Washington rather it's the Dwayne Washington or DeAndre Washington, the Oakland RB. At least he's healthy and playing. Riddick is good for a PPR league, unless you know he's out for several more weeks hold onto him. You're better off with Ajai, who has played well for a couple of weeks, than either of the Washington's. Right now Ajai is a top 20 fantasy RB (pts/game standard Yahoo scoring).