Land Rover Repair: transmission oil temp sensor light, transmission failure, sensor light

Question
The trans oil sensor light is red. I had my fluid changed and a conditioner added and drove 20 mi and light came back on. I have been expeiencing some vibrations in my ride for a few months, I had a driveshaft replaced a year ago.
The mechanic believes i need a new transmission. The car only has 56k mi on it. We also tried putting a sensor from another car in mine with same result.
My question,is the diagnosis correct and how much should a new/rebuilt transmission cost installed and what kind of warranty should I get.I'm not more than 2 hrs from your shop by the way,so I wouldn't be opposed to having you work on it.  

Answer
The diagnosis of transmission failure based on that light is almost certainly wrong and suggests insufficient knowledge on the part of the mechanic.  I don't know what kind of "conditioner" was added, but if the vehicle were here, we would flush that stuff out, fill with fresh synthetic fluid, fix the light and see what it does.

If the transmission did actually need overhaul we would send it to ZF for factory service as they have a regional repair depot near us in CT.