I have seen at least one report of a TC failure to engage, over on the TTORA main board. Interestingly, it was not an 05+, it was an 01-04, IIRC, and it is one of the ones that has what I would call the "semi-electric" t-case: The one where there is a T-case lever to go into H-N-L, and a button on the side to push for 4wd. The person posting was complaining that when it was wet his truck would not go into 4wd, though the high-n-low worked fine. I don't know if he ever resolved it but it appeared to be some kind of electrical problem, a short circuit somewhere that only manifested itself in wet weather.
I'm sure the electric T-cases have gotten more reliable but I'm still glad mine is fully manual.

When I lived in Laramie I had a friend who owned a Ford Ranger 4x4 with the electric T-case and he had constant problems with the case not wanting to engage or disengage in extremely cold weather.
If you've ever been to Laramie in the winter, you'll understand why this is not a trivial issue!