Should I replace the transmission assembly in my 2001 Sequoia or start looking?

Vehicle has 254,000 miles on it. I took it in because it sounded like it was dragging something. They couldn't find anything wrong. Then a day later check engine light came on and I had trouble getting it in drive from park. Don't know fault code. Sorry. I am climbing the learning curve.

Try to get fault code from the computer. I'd also check the transmission fluid level, just to be safe.