I am willing to bet your tranny cooler crossover line is not opening and letting oil through to the cooler.check this test out to check it.
from thediesel site
The Ford R4100 automatic transmission has this device installed on the passenger side between the two cooler lines. It's designed to bypass the cooler when fluid temps are cold to aid in quicker warming to operating temps. It will also bypass fluid if the coolers are blocking flow for any reason. This device can fail at some time. If you find yourself having higher than normal operating temps or temps climbing out of control while your towing, chances are this little device is your problem. It's an easy install. It takes about 20 minutes and you lose very little transmission fluid. See below how to tell if this is your problem.
*****Important Note: We get questions from time to time about the validity of removing this device to eliminate having the replace it from time to time. This is FOOLISH! While it may be an inconvenience that it failed, the worst thing that it cost you is a little time, a new valve and new fluid. If you removed it and the cooler were blocked, you'd lose the transmission in a matter of seconds.
Q: How do I tell if my bypass valve is faulty?
Read the following answer as how to test the coolers and bypass valves.
A: IMPORTANT! The temps should be measured in the pressure port in the driver side of the transmission body. If you are measuring temps from a tee in the output line, temps will read about 30 degrees hotter than temps read from the pressure port.
Bring the transmission up to temperature. It needs to be at least 100 degrees for this test.
Turn the truck off.
Remove the rear line from the transmission and place it into a bucket or similar container so that you can measure volume of your flow.
Have someone turn the truck on.
At idle (in park), you should measure 32 ounces of fluid in your bucket - in 15 seconds or less. This is an absolute minimum number. A properly working transmission will flow that amount in about 10 seconds or less. You should not see ANY fluid pumping out of the rear transmission port.
If you have the proper amount of flow, your transmission pump and coolers are working fine. SO, if you are running hot and have the proper flow (with no fluid coming out of the rear transmission port), you most likely have a mechanical problem causing high heat. This could be torque converter slippage or one of many other internal problems. You may have to see a professional at this point to help diagnose the problem.
If you do not have proper flow, OR you have fluid coming out of the transmission rear port, you either have a cooler blockage, OR a faulty cooler bypass valve. Here's how to tell.
Trace the forward transmission line to the front of the truck.
Remove the line from the 1st place it enters a cooler or the radiator (if so equipped).
Now, do the same flow test checking flow at this point.
There are two outcomes.
The first is that you now have enough fluid flow- this means your cooler /coolers are partially blocked. To determine which cooler is plugged you would perform the same flow test through one cooler at a time.
The second outcome will be that you don't have enough flow even with all coolers disconnected. This means you have an internal pump problem.
IMPORTANT! The outcome of this test assumes that you have NO fluid exiting the rear port of the transmission. If you perform a flow test with the coolers disconnected and you get fluid from the rear port on the transmission, your cooler bypass valve is bad.