Hey Learning:
I'm the researcher & the female in our duo. I was sort of hoping that these men would help you out. But I was waiting for more information from you. What year is your MS, what weight GVWR is on the tag, what year is the F250, what weight GCWR on it & most importantly what is the 5er towing capacity?
Reasons are: the bucking could be one thing & locking up of the brakes could be because you have a MS that has disc brakes & the F250 brake controller isn't compatible. Or like us, something in the bed of our truck pulled out the pin lock & we were stuck in a turn. To help with your research, Trailer Life's annual towing guide from 1999-2007(2008 Ford)
http://www.trailerlife.com/output.cfm?id=42175
Been there, done that. We went from a 16,000 lb OR 5er pulled by a 1-ton to the 18,500 lb ES 5er & F450. But not all at the same time. We kept the ES as light as possible until we could find the truck we could afford & would do the job. Took us well over a year.
Vickie