Your transmission could need replacing.I have not seen much success with getting a rebuilt or used one.Your best bet is to get a REMANUFACTURED(shop will call it reman)one.Since your levels are good.There is also a possibility of the gear speed sensor being bad.I am not sure how far they would have to take apart the transmission to get at it.Also,your best bet is to go to a AAA approved shop to get it checked.PLEASE do not go to a specialty transmission shop.AAA shops have high standards to retain their AAA sponsor and would be much more honest and reliable and are easily quite capable of doing the job,most likely at a lesser price too.If you have any questions or need more help,just comment here and I will get an autolink to your post and reply ASAP.I really encourage responses so I can follow up properly as I know where you stand and asking someone else restarts all the explaining because we cannot see other 'Expert's' responses.Good luck,Greg