Hate to say it but yeah take it to a transmission shop. It may be something simple right now like a loose or clogged vacuum hose or line but that could quickly turn into something way more expensive. I'm pretty sure that old of a Ranger would probably have a C-4 transmission in it. They are fairly inexpensive to repair if it does need rebuilt. Does the trans fluid smell burnt, look black in color instead of red? Or does it have small pieces of metal in it? If so drop the pan and inspect what is inside there. If you see pieces of metal or chunks then you'll definitley have to take it to a transmission shop. Hope this advice helped you out.