Why wouldn't my 4x4 high work but my 4x4 low works great?!?!?!?

2004 Screw Lariat 4x4
switch to 4x4 high = nothing happens, occasional blip of dash light but thats it.
switch to 4x4 low = works great, just like new

 

Im having the same Issue with mine 2004 FX4 but whats wierd is even thoe that light does not stay on it seems to work . I had to use it the other day with the snow we had on the way to work and when I started it back up after my shift the light stayed on but as soon as it took it off and tried to use it again the light did not come back on LOL .

 

I think the light is faulty on our f150's for 4x4HI...as my '07 Screw FX4 is the same way. 4x4low works and lights up. 4x4hi just blips the light when selecting 4x4low.....but I had to use 4x4 hi yesterday and swear it felt like it was engaged and I swore I could hear it.

 

It seems like it is working but something just isn't right. Driving home yesterday my truck started to shudder for about 10-15 seconds.....just cruising down the highway and it did this. This morning I heard a whooshing sound directly under me followed by a loud clunk......again, not doing anything but cruising along.