I have a 98 F250 Supercab XLT. 4wd with 3:73 gear ratio. twist **** 4wd on the dash. Truck runs great but when i switch to 4wd it makes noise and occasionally will pop out of 4wd. The noise starts at almost no speed. I can hear it at less than 5 mph. and only gets worse as i increase speed. (sounds like a bad wheel bearing) - But only in 4wd/ nice and quiet in 2wd. I used sensors and ear phones to pinpoint the noise. It appeared to be coming from the front differential. I opened the pumpkin found clean fluid and no metal bits or any visable unusual wear. I changed the front diff. with another used unit. The noise is still there strong and unchanged. The wheel bearings and axles seem to be allright. Im not sure where to go from here... Any ideas?
- Does this year front diff. have any known problems?
-Anyone come across this problem know what to do?

 

Have you checked the front drive shaft?

Had a similar noise in mine and thought the worst. Surprise to me it was a heat shield barely rubbing the front shaft. Would not explain the popping out of 4wd, but was very noisy in 4wd and quiet in 2wd. Just a thought.