Not exactly something I did today, but over the course of a couple weeks.
I got a CEL, and P0700 and P0740 codes came up. From research here and elsewhere, it seemed that it could be anything from low or dirty trans fluid, a sticky or failing TCC solenoid, or even a bad torque convertor.
Being I've got 160,000 miles on my LJ, if it was more than a fluid deal, I was preparing myself to go with a rebuilt trans and convertor. Being I needed to get my Jeep smogged sometime in the next 1-1/2 months, I was kind of up against a deadline.
I started by changing out the fluid and filter, but 100 miles later the CEL came back on with the same codes. The trans really didn't seem to be acting poorly, the shifts were fine, but I think torque convertor wasn't locking up at times.
I gambled and decided on another fluid and filter change. So far, over 600 miles without a CEL, and I got her smogged. I'm still prepared to replace the trans at some point in the not distant future, but all seems well for now.