unrealIts broken. Sorry, reverse only for you in the future.
Seriously though front diff issues are simple. Its either electrical, fluid or broken parts. Start by changing the fluid in the diff. Its cheap and removes most problems. Second make sure that the front diff is locking into 4x4 when the rear wheels slip. This is done with a meter. Diff should be getting 12v all the time when switch on. If it isn't, then you have an electrical issue. Does your speedo work? It has to work for 4wd as it runs the power through the speedo.
Out of curiosity , how do you know the switch works? By pushing the yellow override button you are forcing the front diff to be energized. If you have power and clean fluid and the switch works they you have an issue with your cage inside and you have to take it apart. There are literally a thousand threads here about no 4wd issues.
The search button is your friend. Also more info is the best way to get help. The whole "its broken, whats wrong"? approach is very rarely successful.
Give us information so that other can guide you towards a correct fix.