Trans. output speed sensor?

'99 f350 4x4 with auto trans. 7.3L

this summer i have had issues with my truck. i am getting a code for the trans. output speed sensor failure. every time this has happened the truck is cold (just started up) and trying to pull up an incline/under load. if the truck is warmed up it does not happen. i can drive the truck everyday and it has no issues (it is flat here). when i was staying in a cabin up in arkansas the drive to the cabin was a pretty good incline. when i'd get up to go fishing in the morning, i'd go to take off and the truck would bog down like it was in limp mode and the OD off light on the shifter would flash. every morning i was there this would happen. never when warm. when i have taken the boat to the lake here at home i get there fine but when it is time to leave i back the trailer down the ramp and turn the truck off and go load the boat. when the boat is on the trailer and i crank the truck up to pull out the same happens. this last time i didn't think i was going to get up the ramp. i kept having to stop and let the truck run. i guess my question is can the sensor it's self cause this or am i looking at an issue with the trans. it's self? thanks for any help.