Assuming you have a DII, the trans temp sensor is a known fail item.

 

Thanks for the heads up!

 

Yeah my disco II was doing this a lot, Ive towed 2500-3000lbs about 1600 miles so far. Had to stop about 8 times so far, did a fluid change mid trip and still I am stopping about 2 or 3 times to let it cool down. It was working great after the initial fluid change out, hit the hardest leg of my trip at night through utah mts and she really hit it like a champ.

-Viscosity I think breaks down in the fluid pretty fast while towing, try night drives or like people said a cooler. Do not push the high range in acceleration, hit hills going well above the speed limit (20mph more) and dont floor the heck out of the inclines.
-I try not to go above 3k rpm as a rule. To cool it, going downhill, shift to neutral. This allows the fluid to cycle through the cooler and stops friction for awhile.
-Using the gear shift doesnt do jack, the rover does an ok job by itself.
-Cruise control is a joke while towing, dont use it, no matter how long of the drive towing is a user controlled job unless the road is 100% flat.

Why land rover built the coolers sandwiched between the d*mn AC condenser and Radiator is beyond me, horrible design. If I thought I was going to ever tow after this I would move the location of each to the front fog light wells and modify it, would be 10x more efficient.

Also my fuel pump died from towing, the demand from the fuel usage and the old age of my disco II (same mileage as this one) caused it to flip out on an incline. Left me stranded, so be ready for that.

Despite the tow package and pitch of the disco II, I will not tow anything like this ever again except from the hardware store/lumber yard and back, its too much of a hassle. My temp sensor, and I suspect yours, is working fine its just the driving habit/discovery II itself.

I've made too many excuses for the truck's lack of usage, so I take all specs and divide by 2 for the ratings and that seems to be the actual. If you really want to tow, turbo diesel, which there are a few disco IIs with that package in the USA but rare thanks to emissions law. Still their trans coolers are the same story. I just couldnt afford a superduty or cummins diesel truck.

I love my discovery II, but she isnt made for towing what the specs say, most trucks aren't, the specs are the max at sea level, on flat road, average temps, etc.

 

How many miles on this truck and when was the last major service done as well as the suspension, be it shocks and coils replaced?
Looks like you don't know too much about your truck, especially the tranny temp light that will also come on when the t/case over heats, so when did you last change the t/case gear oil?