It's a damn good idea to take a look at retaining him in a keeper league, especially if you can keep a substantial amount of players. After this season, it does look like he will be the RB of the future.
As you mentioned, the Priest is already 31. Priest is a special back, but has always been injury prone. His decline may be sudden and unexpected due to this reason.
IMO, Vermeil will stick to Priest as long as he's healthy and young enough to play well. Let's face it. Even though he got hurt last year, he looked anything but 31 before the injury. Vermeil has always preferred one starting back, so I don't see him giving LJ that many carries (enough for you to start him in a league) this season. However, I don't expect more than two years out of Priest...making LJ a good keeper candidate.
"The son of Bobby Bowden once again proved he's as predictable as a Julia Roberts' movie.
And upon further review, just as sappy, too."
-Naples Daily News