Transmission Problems?

Yet again I am confused about my cars recent behavior. The other day I was driving home (25 miles from work) and kicked my cruise off before the stop sign and my car made a weird thud followed by some shaking when I pulled up to the stop sign. I tried accelerating and the car would not go faster than 20mph and felt like it wasn't shifting into second gear. So I drove the 1/2 mile to my house going 10mph but before I got there the Check Engine Light came on so I gave my uncle a call to bring his code reader over to see what it was spitting out. It came back with "Transmission lock stuck" I believe. My uncle said that the car was stuck in 4th gear and that was why it wouldn't accelerate and felt like it wouldn't shift. After he got the code we took it for a drive and it acted fine. A few days later I drove it again and it did the same thing again so I dropped it into first gear and it manually shifted it up to third. The transmission was shifting hard when I was doing this though. Once again I turned the car off and waited about 10mins and drove it again and it was fine. I am wondering if it is something electrical and not my transmission going out. Also, after the first time that it did this my car started not shifting out of park without pushing the shift lock release button. Not sure if that has something to do with it or just another problem. Any ideas would be greatly appreciated. Thanks again everyone :)

It would help to post the year and model Grand Prix you have.

Also post the specific DTC from the code reader.

Sorry about that. It is a 1998 Grand Prix GTP. I just had the engine replaced in it about 2 months ago. My uncle is out of state this week so I cant get the exact code.

AutoZone will pull the code for you

Usually a trans code needs to be read with a scan tool capable of reading GM extended service codes P1xxx. Generally autozone or their competitors don't have the equipment to retrieve these type of codes.

I haven't got the Check Engine Light to come back on since this has started happening. It came on the first time but the other two times it has acted up it hasn't came on.

For example if you get a P1811 DTC which is a PCS solenoid issue causing the hard shifting it does not turn on the SES/CEL light but is stored for retrieval with a trans capable scan tool or one that can read GM extended codes.