I'm having issues with my 2001 Ford Ranger?

I recently drove to the store and got on the gas a good bit. After doing so, the O/D OFF light started flashing. I've had this issue before but this time my truck was refusing to move when I gave it gas. When I had it in drive it did almost nothing. I had to put it in first gear to go anywhere. It would barely move in reverse either so I had trouble getting out of my parking spot. I'm just wondering what exactly could the problem be here? As much as I hate to say it I'm sure it's something wrong with the transmission. Any other possibilities?

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