Engine/Transmission (1998.5 - 2002) Rapid O.D. in and out at low speeds

[Motorhead]

I have just returned from a trip, towing a car trailer. The next time I drove the truck, I noticed when it went into lock-up at low speed and light throttle, It went in and out of OD rapidly, Cycling until I raised my speed. I have seen this on the site before but I didn't find it in the search. Please help. . I have to go to the dealer to get my driveline replaced and if it needs a reflash, I'll do it then. ALSO, I am going on Vacation in a week and will be towing heavy.