I assume your trans is a C6 so the shift points are vacuum controlled. As your truck is a diesel the vacuum source is a vacuum pump driven off the crank pulley. So have you got plenty of vacuum? A good indication is if the power brakes work good. If you have good vacuum this is supplied to the C6 via a modulator valve on your injector pump. The vacuum finally ends up at a modulator diaphram on the C6. So check all the vacuum pipe work. The pipe going to the injector pump and leaving the injector pump and running down the frame rail to the trans. If this all checks out pull the vacuul pipe at the trans and see if any red fluid is present, this would indicate a faulty diaphram. Good luck. I should add that if you have had any work done on the injector pump the vacuum modulator valve may be out of adjustment. Good luck.