It probably lost its 3/4 pack. That's the technical term meaning 3rd 4th gears are damaged due to the loss of fluid. My trans mechanic told me that there's a fluid port that supplies fluid to the 3-4 pack. And that port is designed too small, DESIGNED TO BREAK, after so many miles. That don't apply to your situation but if it will only go 20-30mph its probably not hitting 3rd gear. Time for a rebuild unfortunately. They can replace the 3-4 pack only but most shops will want to rebuild it in total. I found vids on the YouTube.
The original problem was caused by loss of fluid from the transmission due to the cooling line coming loose from the radiator. Automatic transmissions have a fail-safe mode that locks them into a low gear so the vehicle can be safely moved out of traffic and to a service shop.
Automatic transmissions are shifted by hydraulic fluid pressure inside the transmission and loss of pressure is one of the conditions that will trigger the fail-safe mode.
Newer transmissions like yours often will need to be manually reset to release the fail safe mode. I have not personally worked on one from the Yukon, and you did not give the year of the vehicle so I am not sure I found the exact service manual for yours, but one I found on line is at
GMC Workshop Manuals
A scan tool on the OBDCII port will reveal any stored error codes and in most cases is able to do a reset of computer codes that lock systems into fail-safe modes. That may work for you.
If resetting the OBDCII error codes does not work then I would suggest a full reset of everything on the computer by disconnecting battery cables for about 10 minutes to be certain the computer 'forgets' everything it has learned and stored about the vehicle condition and can start again as if it were a new vehicle.
Finally, if both those fail, then I would suggest you take it to a service center for professional help. Running it for even a short distance without fluid may have damaged the transmission internals. No oil pressure means loss of both cooling and lubrication.
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When I said all but 1 car, what I really meant to say is that all cars in the household had broken down except for 1. We have a Nissan Frontier with a blown head gasket. An Infiniti I30 with some type of problem which won't let it start at all. A Volkswagen CC that had a timing chain fail which rendered the engine useless and being that it was over 80,000, miles out of warranty. And last but not least, the 2001 Yukon which I posted on here about which just went up with the transmission issues. For now we have a 12 year old Ford Taurus which is still chugging along. And still waiting on a solution if at all.
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