Ok here is the deal I have an 03 Excursion Limited 4x4 with a 6.0. 204K miles never had a problem. A while back while driving down the interestate there was a sound a clunk and I stopped to see whats wrong. Low and behold my transfer case was hanging out of my truck. The transfer case adapter housing broke. Trailered it home. Got it apart and it had been working its way ended up sheering the teeth inside the t case and the shaft in the trans. I took the t case out of my 04 f250 and the adapter housing and put it all back together no problems. Drained the trans, did a flush changed filter put in new fluid. Now i took it out to drive and when it shifts into 3rd it shifts hard then the overdrive light starts flashing. If you stop and shift through the gears it only does it in reverse and drive. Shut it off and restart no blinking light shifts good till you hit 3rd any ideas, before i take the trans out and replace it with my 04 f250 trans

 

With that many miles it is probably due for a rebuild but check for codes first.

 

ford rebuilt the trans at around 170k. The truck shid fine before the t case adapter broke. Could it be a VSS or tcc

 

Get the codes before over-thinking it.

 

Moved to the 6.0L PSD forum.

Stewart

 

It could be almost anything. If you rebuild the trans you might still have the same issues. The ONLY way to know what's wrong is to get the codes read.