Maybe try backing up and alternately tapping the steering brakes?
So, what is the dealer saying?
Many times the seal or o-ring is on the inboard part of the shaft so the rusting can happen on the outboard portion. Keep lubing and working on it as it may take a while for the lube to work in.Before throwing in the towel for a warmer day I put a 10" pipe wrench on it, just enough leverage to get it moving. Lubed it best I can, will PB blast it again next warm day.
Looks like the other actuators (i.e. 4wd) enter the housing with some sort of a replaceable seal, where the diff lock is just a rod into the housing. I'm assuming it's above the oil level and thus rusts...