you don't need to replace the seat belts. Remove the "l" shaped piece that is affixed to your door jamb with two small screws. If it's not "l" shaped it's broken. You can find at a junk yard for next to nothing.
To get a better idea of how it works pull the rear seat out (it's very easy) and pull open the plastic "sail panel" it's probably just mis-aligned. Btw, cleaning the seat belts themselves doesn't hurt either; especially in the triangle shaped part above your head where the belt slides through. It accumulates with the ramdon dirt and grime of a seat belt. Over 20 years it's probably pretty dirty. This too, may impede your belts ability to retract properly.
Good luck.