I have a 1999 150 5.4 4x4. Today on the highway It would only go 45mph in OD. The OD switch is blinking. With the OD switch off it shifts 1st and 2nd gear fine. It does not seem to shift to 3rd and if I turn the OD switch on it goes into a neutral situation. I just put this tranny in less than 40k mile ago and it had 29k on it. Have not had the codes checked yet.
Any suggestions to what it may be?

 

You need to check the codes.

 

can I have it done at a local parts store or must it be a repair shop?

 

Parts stores don't have capable scanners. It needs to be a shop.