Set off this morning to buy a mustang, However i never got there.

When the truck tries to shift between what i believe is 2nd and 3rd it just slips then engages with a thump. Flashes up Trans Malfunction and O/D light flashes.

If i take it out of O/D it drives fine.

Serious problem or easyish fix? I guess she just wasn't ready to be traded on yet and wants to hang around a bit longer.

 

It's hard to say without knowing what codes were stored when the lights flashed. Even if the lights don't flash again the codes will stay in memory for about a month. Get them read and then we can offer opinions on what's wrong.