Ok, there are several possible causes - transmission fluid pump failure, cracked band or servo piston, valve body failure, input or output shaft failure.
The only way to know which has failed is dropping the transmission and tearing down for inspection of the components.
The quote you were give for a new transmission is on the outrageous side.
The transmission part number is *****
Here is a link to it and you can buy it for around $2300
Labor is 6.2 hours to replace it, so around $620 in labor costs.
Total about $3000
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