Transmission shifting roughly

The thunking noises for the back of the car wouldn't be related to the trans-axel transmission. If your problem is hard shifting and the fluid level is at the proper level then I wouldn't recommend paying to have the fluid changed, chances are it won't make a difference. Are there any check engine codes on your dash? This could indicate a problem with part of the transmission. Unfortunately, your car is starting to get high in mileage and if the transmission wasn't serviced properly every 60,000 miles or so it may have permanent damage. Check your fluid level, check any OBDii codes. You may have to replace the transmission. Changing your transmission fluid is just like changing your oil, every 3,000-5,000 miles you change your oil, every 30,000-60,000 miles you change your transmission fluid depending on the type of car.

The major local auto parts stores will read codes for free, like you said there aren't any codes displaying on the dash so it's a good chance we can rule out any faults there. If you can get a good flashlight under the car you may be able to see something easily noticeable that looks disconnected in the rear end. Possibly part of the suspension or exhaust system has broken off, It could be a number of different things. Going back to the transmission, keep in mind that keeping the fluid at the proper level and changed in accordance with the manufacturer's recommendation is very important in its life. The car had relatively high mileage when you got it so yeah, it's hard to say if it had ever been changed. Good luck and let me know if I can help with anything else.

Hello, I'm Danny.

Since you don't know the vehicle history you should assume the car was neglected. The mileage isn't all that high so the transmission might not be in all that bad of shape. Use a white paper towel to check the transmission fluid level again and look at the color. Is it brown or dark in color? Smell the fluid as well. Does it smell burnt? It normally should be clear red in color when fluid is in good shape. You mentioned checking vehicle codes. You don't need to use a code reader or scanner if you don't have one because a paper clip or piece of automotive wire will suffice. Here is a tutorial showing how to check codes on your car which can include transmission codes:

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

Also, you mentioned a clunking sound. When it comes to sounds they are hard to diagnose without being by the car in person so what we ask is to use your cell phone to make a video and upload here for us to listen to. After you check for codes get back to us with any information and we'll go from there. Hope this helps and thanks for using 2CarPros.

Hello again.

Okay, then just get back to us after checking the condition of the fluid and checking for codes and we'll go from there. Hope this helps and thanks again for using 2CarPros.

Danny-

You're welcome!
Thanks again for using 2CarPros and we hope you will use our site again in the future.

Danny-

Hello again,

Since this is not transmission related the proper protocol is for you to start a new question for an OBD1 issue. Here is the link:

https://www.2carpros.com/questions/new

Be sure to include all your vehicles information and the OBD1 question. Once we receive your new question we will be happy to help. Hope this helps and thanks again for using 2CarPros.

Danny-