HELP! 2006 4L60E transmission diagnostics

I have a 2006 Sierra 2wd 5.3 liter with a 4L60E. Transmission was replaced about a year ago with a used one, of course warranty is out. New fluid and filter (fluid in torque converter was topped off, not drained). All has been well until a little over a month ago during a normal takeoff a clanging sound that felt like jumping teeth then back to normal. No slipping in any gear, able to do occasional strong takeoffs etc. Checked fluid level, no burnt smell. Noise occurred a couple of times randomly sometimes after applying gas from a coast, sometimes from a stop. Shifts to drive or reverse from park were normal. Shifts while moving normally or under strong acceleration are normal except the 2 or 3 times I heard the noise. (Always under normal to lite acceleration) This all over the last month until yesterday I pulled off from a stop and the clanging, teeth slipping noise and no shift to second at any rpm. Still no slipping. Reverse is also working but nothing but first and reverse. Shifting manually has no effect or starting in 2nd or 3rd. I assume it is in "limp home mode" but no codes or service lights until a "Transmission Hot" warning came up on the info screen and I immediately stopped and called for a ride home. Next day checked fluid level again and checked for burnt smell, all seems normal. Waited for low traffic and limped home at 20-25 mph several miles with no noise or overheating but still stuck in first. To me it seems to be something other than just a fried tranny. Valve body or solenoids maybe or cracked Sun Shell, anything other than another replacement or rebuild would be great. Any help would be greatly appreciated.