My rear gas tank is stuck on full?

I have a 92 Ford f250 4x4 xlt and I recently filled my rear tank all the way and now it hasn't been stuck past the full mark

Keep track of your mileage, so you know when you should probably fill up.

Bad fuel sending unit, they have a gauge that goes in the top of the tank that measures the amount of fuel in it, over the years those can stop functioning properly and stick in certain places. You can either drop the gas tank and replace it. Pull the bed and replace it, or… Deal with it lol