My first question would be, did you do anything to the car before the symptom showed up?
Second, it does seem like a transmission issue. There are a lot of simple things you can check before going down the rabbit hole like, have you checked your trans fluid levels?
Make sure to check the governor control cable adjustment (Workshop Manual G:05:02). It’s an easy thing to check, and your shifting will be off if incorrectly adjusted. (Probably on all gears though).
After that I would check transmission wiring for any signs of burning or melting, followed by verifying vacuum connections to the transmission.