Transmission

Hard to say but check the ATF level.

Speed sencor

Check your transmission fluid and filter, chage filter at least and fluid if dirty or just old. Then just saying make sure your rear end fluid is good. It may be your rear end and sometimes it can feel like transmission. And yes like the second gentleman said Check your speed sensor, those can go or start to act up before it goes and will throw off your Engine / Transmission Synced Mode , (It doesnt shift when needs to can be off by 2-3, or 4 - 5 mph +/- . Is it shifting hard..? Like driven it and you just past the speed that it should shift then 2 or 3 mph after or so +/- it finally shifts but really hard... like boom, the rear end feels like its jerking. Also... just throwing it out there after all that if still doing it.. check your brake pads, rotors etc... also just make sure your brake fluid is nice and clean.. that can cause issue. Nothing to thst degree but might as well make sure nothing is locking up back there just enough to mess with things but not really notice it, like no burnt smells or it dont feel like your driving around with Emergency Brake on. Just making some Observations. Goid luck. Let us know qhat happens please, if you think of it. Thanks and Good luck.