Chevrolet: transmission issues, transmission issues, highway speeds

Question
My 95 cavalier Z-24 with 126,000 miles has had tranny issues since 54,000.  Right now it makes a horrible thump in the transmission when I shift from drive to reverse (visa versa) and give it some gas and also it seems to shift in and out of over-drive at highway speeds.  

Answer
HI;
   I would say the first thing to do is check the tranny/moter mounts and also the  joints /axels etc. that is not a good thing. or you could go to a tranny shop only a good place like aamco , mavis, and ask them for a free check up or a $ 29.00 check  on the tranny / noise   as this is the best way for not spending a lot of cash. and you will know what is wrong.
good luck
roger