If you have not checked the front turn signal bulbs and sockets on your 2005 Chevrolet Uplander please do so before anything else. If bulbs and sockets check OK then you will need to test to see if you are having a problem with the power or ground side of the circuit. A test light and some basic electrical knowledge will be necessary to check this. From what I can see on the wiring diagram the turn signals are controlled by the Body Control Module and are completely separate circuits meaning each side may need to be diagnosed individually.
Having the same problem with my 2005 Chevy Uplander. Only difference is my Right blinker will not work and the left one does. Sometimes it will work for a few days and then it goes off again. What's up?