2004 F150 4x4 (grinding in 4h4l, fine in 2h)

[tfrazee]

Group, I have been reading many posts and it sounds like my problem is a little bit different than what I've seen. From what I have learned, in 2004 1/2 and up models had a vacuum motor that did the shifting at the axle to engage the axle-halves to the pumpkin. And this was designed for a fail-safe operation as if/when the motor failed it would stick in "connected mode". Am I correct? If I am, then my problem confuses me. Here are the symptoms. 2H is fine. Works like a champ. When I engage 4H (on the fly or stand-still) the light comes up and I hear/feel a thunk of the xfer case shifting underneath. When the wheels move at the same rate (on wet pavement or on snow) its fine, but if I goose the motor (which in turn should "goose" the wheels) I hear a grinding sound. Almost like some teeth somewhere are not catching each other. Here in St. Louis this weekend we got our first good snowfall. I was as stuck as the geeks in their mom's minivan. All indications of being in 4H were there but when it mattered (like when giving the engine some gas...) it was very obvious that only the back two wheels were working. Do I have a vacuum motor problem? a transfer case problem? I had the transmission oil replaced this summer. Does the transfer case get its oil/fuild from the transmission? I only ask as the whole 4x4 function on my truck has felt a little funny to me since then. Might be just a coincidence. Any help or ideas would be awesome.

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[glc]

No, the transfer case has its own oil.