I have a 99 Blazer/Jimmy that had major slipping and NO Error codes or lights. I replaced with a rebuilt tranny 4L60E and converter. Now the light came on and I get a surge at 65mph. If I set the cruise or just hold the pedal at 65 mph the tach goes from 2100 rpm to 2000 rpm about every 3-5 seconds and you feel this surge (as I call it) It only does it in OD. The place that rebuilt it said it was not a trans issue it was something telling the torque converter to lock and unlock. that was my surge. My problem is I had no light and no surge with the old one just slipping, now I have more/other issues than I started with. Any ideas on something to check?

 

This might give you an idea.
https://blazerforum.com/forum/diagnostic-trouble-codes-dtcs-49/p1870-explained-18227/