1996 Chevy Cavalier Drive

Check the shifter linkage, make sure it is not kinked or a bracket is loose. Move shifter (engine off) while someone looks at connection to trans. Make sure it is moving the full range.

If equipped with detent or kick down cable, check cable, it may be broke, or way out of adjustment.

Remove trans pan and change filter and add fluid to full.

If nothing above works, take to trans shop, some are offering free diagnosis, but generally when a trans starts slipping, it is due for rebuild.

You can also consider replacing trans with used one from salvage yard (especially if you are doing it yourself). The downside to this is that if replacement trans turns out to be bad, the salvage yard's warranty will only replace trans, not labor to replace.