Checked with dealer?

 

Ford

 

Like everyone else said. Ford.. First comes the harsh shift then if ignored and especially if truck is used for towing, then comes the shift flares.. Valve body swap will fix if it was just the harsh shift (Failsafe mode).. Once they start slipping, sometimes its too late.. Drop pan and check for a ton of clutch material.... Pull filter and look up in there at your 3-5-R clutchs... IF you can see a lot of the inner drum splines then itll likely need a rebuild or a new unit.
Had a customer I told he needed to fix the harsh shift ASAP, or its gonna get more expensive. Had this reason and that reason why 'lets not do it now'.. He Drove it like that for another 2 years till it started slipping then brough it to me for the valve body and actually was cussing at me when I told how much more it was going to be now.. I sent him screen shots of when i told him not to keep driving it like that.. Guess what.. I put a new Ford trans in his truck... Been doing fine since..