I towed a lot with a 4L80. I ran it in OD when it would stay in gear. I pulled 14,000 lbs a couple times. (It had a supercharger) The biggest concern like said above is heat. The TC on these doesn't lock up until you get up to about 60 mph. If you fall below 60, you lose power, and it gets hot.
I found this on another site, and did it. This mod is awesome. I could manually lock the tc up at 35 mph. My heat issues were GONE. I changed my fluid often - and every time it got above 230.
"I found out that if the thermistor in the transmision is working correctly and it senses a temperature over 284 deg F then the trans will go into hot mode, freeze the shift adapts and record in history the code for a hot trans. It will not turn on the service engine soon light.
So what I did was to wire a switch so that a 50 Ohm resistor could be switched in and out of a parallel circuit with the transmision thermistor. What this does is trick the PCM into thinking the trans is close to 300 deg in temp so it works the TCC in hot mode. The goal is to make it think its hot without thinking that the thermistor circuit is shorted. Also I wired in an LED to the TCC solenoid so I could tell when the PCM was calling for the TCC to be locked.
Here's what happened with the switch on. The TCC will lock with the trans shift lever in second when it exceeds about 25mph. Also the TCC is used longer, more frequently and under higher throttle in 3rd and 4th. It still goes off when a shift is made. Also when slowing down the TCC will stay locked until you let almost completely off of the accelerator pedal. Shifts were still smooth.
I did see one thing though that needs to be watched out for. In some situations with a very heavy pull such as accelerating hard up a hill the TCC may continue to slip until it can catch up. What you need to do in this case is slack off the throttle until the TCC completely locks up then reapply throttle. It will then hold the big load."