2000 GMC hard 1 -2 shift

My truck with 5.3 engine 4x4 1/2 ton 118,000 miles all of a sudden started shifting hard while coming home. The service engine light came on before I noticed the shifting. I thought it was my gas cap as I had just filled the truck. Can I get the code without a scan tool? Any ideas what my problem might be? I need to get to work Monday... Thanks !

Update
I got home and let it sit for about an hour. I went out to try it again and it shifted normal ! Is there something sticking ? Should I change the fluid? Should I use one of those fluid exchange things?

Sometimes you can get a bit of grit or debris in the valve body. Generally speaking, harsh shifts occur because an accumulator (a spring-loaded piston moving in a bore) can get hung up due to the debris. All it takes is a tiny piece of solid material in the wrong place to make an accumulator stick.

When you shut off the engine, the fluid pressure drops and the grit is swept away by the draining fluid.

Drop the transmission pan and drain the fluid. Clean out the inside of the pan and magnet and change the filter.
Take a good look at the debris in the pan. A modest amount of black/grey muck is normal. However, lots bright sparkly flakes in the muck or fluid is not normal and indicates severe wear on some metal parts due to a problem.

Replace the gasket and pan, and fill with clean fluid.

Don't bother with one of those trans flush (fluid exchange) things. They are not good at removing debris from the pan.

Thanks !

I drove my truck for 3 hours today and stopped and restarted many times without any shifting problems ! I stopped at Autozone to read my check engine code and it was a transmission error. As I was driving the check engine light went out and has stayed out. It had to be just some debris. I am going to drop the pan and change the filter and clean it, THANKS !!!!!!