At 136,000 miles the TC case chain started slipping on my wife's 2004 2.5. That may be the case with your truck(no pun intended). I ended up replacing the TC and the rear driveshaft(bad u-joint) and all is back to normal.

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1975 2002A
1995 318ti Club Sport
2004 X3 2.5
2009 328i
2007 Mazdaspeed3
1999 Wrangler Sahara
1996 Speed Triple