i have a 99 f-150, 4.6L, 4x4, auto w/137,000 mi. while in reverse it will make a squeaking sound. but if i shift to neutral while rolling backwards it stops. if i am in a low gear (say going down a hill) it will make the same noise. it is kinda like a suspension squeaking type sound, but it is not the suspension. could this be the u-joints.

also, i tried to shift to 4x4 the other day and it shifted but nothing happened. it is not electric 4x4. it is a lever shift on the fly. the 4x4 light will come on in 4hi and 4lo but it remains in 2hi. even in 4lo it still runs in 4hi. what could be the problem.. any help is greatly appreciated...thanks

 

Never heard of a u-joint squeaking. But when they are going bad it will clonk every time you put it in gear.
Sorry can't help with the 4X4 question.

 

Don't have an answer for the 4wheel drive issue, but u-joints can diffently squeak. Usually it will squeak at the same rate as the driveshaft turns. Hope this helps.