Here is the link to the T-Maxx 2.5 Manual or look in your copy if your truck is new. Page 36 is the 2-speed adjustment page...
http://www.traxxas.com/PDF-Library/4910-08_manual.pdf
You probably need to readjust the screw that changes the shift point...
If adjustment won't fix issue, contact Traxxas for assistance. I hear they are great on warranty repairs...