i have a 99 f150, 4.6L, 4x4 w/136,000 mi. i tried to shift into 4x4 the other day and nothing happened. i went to 4lo and it remained in 2hi. when i shift it, i can hear it clicking like it is engaging and the 4x4 lights will come on. a day or two later i tried it again and it would not go into 4hi but when i put it in 4lo it went to 2lo. which was different than before. any ideas of what i am dealing with??