^^^ Pretty much hit the high points - how often is it used/ engaged and how old is the fluid? I have tried to engage mine at least once a month just to ensure it works and keep it moving. The fluid is important because if that stuff is old it will not work like it should and it is not good for the diff.
Are you talking about trouble getting into 4hi or 4lo? What kind of transfer case do you have?
Good maintenance. Well at least it works in a kooky kind of way. Any pending codes by chance?
mine had the same symptoms as yours. I never knew if it would engage or not. Finally changed the diff fluid, TC fluid, and rear diff fluid. Didnt change anything. after several months it would NOT go in to 4wd. I had to change the ADD on the front diff. Costly little rascal but it immediately corrected my problem and after about 9 months it is still working fine. Be advised that the 01 and 02 are electric not vacume
What was the cost on that.
Have a buddy in service department at Toyota guess it would be easier to take a day off and drop it off with him so he can diagnose the issue. Just hate not able to figure this out my self. Small problem to have but regardless it's still a problemDo you have an FSM to chase this down with?
I found this under "Shift from 2WD (H) to 4WD (H) impossible":
List of possibilities to check in this order -
1. 4WD fuse
2. Wire harness
3. Vehicle speed sensor
4. 4WD position switch
5. 4WD indicator light
6. Actuator assembly
7. A.D.D. control system
8. 4WD control ECU
9. Transfer assembly
While stopped, try engaging 4WD with trans in neutral then put it back on Drive and start rolling to see if it works...