4x4 doesn't always engage on my 99' 3.0
 

I have a 99' 3.0 4x4 and the 4WD doesn't always engage. It only seems to engage i'd say 1 in 5 times. When I start the truck drive around and put it into 4x4 high i can here a motor like sound trying to engage the hubs for about 5 seconds but it wont catch, the motor sounds stops and then the 4x4 high and 4x4 low light will start blinking. It isn't completely broken it just does not engage all the time. Any ideas as to what this could be ? The truck has 85k

thanks,

anyone ?