I had a used transmission put in and same thing is happening

It's hard to know what is going on here, this mechanic seems to be guessing and I'm not sure how much actual diagnosis they're performing. I would want all the diagnostic info from the previous and current repair attempts, and I would probably get a second opinion before replacing any parts. Is this a transmission shop?

I had a 1997 chevy 1/2 ton PU truck that the altenator went bad in. The first thing that happened was the tranny would not shift correctly. I thought my transmission was going bad, but it was the alternator. I understand if the on-board computer isn't getting the correct amperage, The transmission, among other things, will malfunction. Hope this helps some.

it was likely, ever only the speed sensors. Do not go back to that same mechanic. Search your faults online and you will likely come to the right answer. Find more then five sites with the similar solutions and you can be fairly certain you're on the right track. The speed sensors in this situation cause a ton of shifting errors, from sticking in gear to hard clanking/shifting. To find out if your transmission is failing, you need only look at the fluid on the dip stick. If the fluid is red/pink you're likely ok, where as if it's black or has any amount of contamination in it, you would then be looking at a failing transmission. I'm skeptical that the mechanic in your situation didn't at least know to replace the sensors. Seems like he totally missed the easy fix, searching for the big bucks. I wonder if he even changed anything.