I have a 1999 Dodge Ram 1500 pickup that don't shift until its at 20mph.?

I just bought it as a 2nd vehicle for my business. You know, so the young kid who works for me can run for equipment and supplies for the jobs we have. So it doesn't need to be a flawless, just needs to run good. But the tranny doesn't shift till it reaches hi RPM at about 20mph. If I have to replace the tranny no big deal, I paid $500 for it. But if I can fix what I have, bonus. I'm thinking that the kick down cable needs to be adjusted or replaced.