Little difficult understanding your description but I'm pretty sure your saying the truck is shifting too late?? First step is to properly check and see if the fluid is low, if not take a look at the fluid, does it smell burnt, look discolored? If the fluid is low top it off, if it smells burnt or is a funky color, prolly means you have done damage to your tranny and its only a matter of time until failure. If all this checks out, best to take it to a tranny shop for some tests, today's trannys are electronically controlled, with many sensors and many things can cause this. Best to have a respected tranny shop test it for you. Changing to synthetic will not solve this problem, it may appear to lessen the problem some, but either way if you continue driving it like this the tranny will have a very short life.