PhD.-Application; How to deal with reference-letter requests when being out of academia for 2 years?
I suggest asking your old writers or others at the institution for new letters. Treat it as a chance to catch up with old professors which is a good thing to do for any prospective academic. 

The old letters have less relevance and new ones, while still dealing mostly with what you were like two years ago can give some assurance that you've been active in the interim. 

Two years isn't that long. But if the gap gets much wider people will start to forget you. This is a good time to renew old ties.