I know this is an old thread, but the same issue is going on right now...
I've had Bilstein 5100 leveling shocks on order for the front of my 2011 FX4 since September 1. Bilstein keeps giving an estimated ship date, and when that date comes around, they push the ETA another 3 weeks or so.
Now they are saying Jan 10, over four months on back order. Unless they push it again (which has already happened four times).
Evidently this is an ongoing problem with Bilstein. Unfortunately there really isn't any other option I'm interested in.