OP, two things. The front ADD actuator has a TSB now. It looks like a part change but I have not been able to figure out yet what is different. It had a bit of a reputation for taking in water but you said that yours was ok.
The actuator in the transfer case has had the same problem on occasion. There was a part number changed on it in 2008. Its a mega expensive actuator and generally the transfer case must be removed to replace it. What a number of guys have done is take off the actuator's housing cover, cleaned it up, and tried to free it up mechanically. The transfer case actuator has become a use-it-or-lose-it deal. Apparently the grease goes to hard goo especially if some water gets in there.
Obviously check the easy stuff first such as corroded terminals an broken wires.
Once in a while the 4WD switch itself on the dash goes bad. Also the 4WD computer above the glove compartment is in mix but I have yet to read of one that has gone bad.
If I get a minute today I will come back with an edit and give you some links on this general problem.
This is directly from a post I did on another forum. Something in these links should help you:
Check out Post No. 4 at this link: http://www.tacomaworld.com/forum/2nd...ot-engage.html
Please ignore all those other comments in that thread about those lines being vacuum lines. We have already established that they are not. They are vent hoses and vent hoses only.
Here is a good thread about the problem you and others have had and some good diagnosis: http://www.ttora.com/forum/showthread.php?t=175572
Another thread; got a hammer? Check out Post 10. The rest of the thread is informative too: http://www.ttora.com/forum/showthread.php?t=158372
Clean out the front ADD actuator. See this thread and especially Posts 18-24 by tacosupreme
Thread about general problems and failures with the front ADD actuator, transfer actuator, confirmed replacement of front ADD actuator fixing the problem and other fixes: http://www.expeditionportal.com/foru...p/t-90151.html
Good luck, and please keep us posted as to what you find so others can benefit.