Truck is a 2005 5.4 FX4 5.4. I went to use my 4WD today cause it's snowing.
Turned to 4High switch position while going 25 mph, and got no familiar thunk, but the "4 high" light came on. When using higher amounts of throttle I heard some banging so I backed off. Later I got stuck and had to go down to 2wd to rock myself out of a pile of snow because of noises when 4wd was engaged. Later I tried 4 low, the 4 low light came on but I think I still was in 2 high by the way the truck acted and ran. I also tried using 4 high from a start and got more bad noises which seem to be from the t-case.
I did a transfer case oil change this fall with the recommend ATF and used 4wd a couple of times but never in low traction situations.
What should I check?

 

I have the same problem I have a 2004 lariat with 13600 miles on it It started sat morning I put it in 4high when i got out of the garage I took off from a red light and the rear end broke loose front wheels never spun This has never happened before I will be seeing the dealer on monday I am sure glad i bought a ford esp premiumcare warrenty before my 3 year mfg warrenty ran out in oct

 

I am having the same problem. The only difference is when I put it in 4L, it does run in low gear. Probably just something like 2L. I will be seeing my dealer as well come Monday morning.

 

Good luck with your visit Hope it turns out well

 

I found another post with info on vacuum actuator location and basic inspection for leaks and operation. Only problem is it's the previous generation truck. Is an 03 similar to an 05?

 

From what I have learned, Ford redesigned the front hubs so if there was a vacuum leak, it would engage the axles. Basically meaning that if it broke, you would only suffer from poor mileage and increased wear and tear. I'm sure there are folks on this list that know more than I.

 

If the seal in the IWE leaks it could suck up some water and cause your symptoms. I was looking for the TSB on this issue in an other thread but I could not find it.

JMC