Sounds like moving over rumble strips? Sounds like the torque converter, but it may not be bad......just old fluid. Before getting into anything too complicated, I would have a technician evaluate the condition of the transmission fluid. If it's not too hateful, have a BG transmission flush done on the vehicle. Ford has a general Technical Service bulletin that recommends the flush for your problem. The TSB
was issued because dealers were misdiagnosing bad transmissions and
replacing them under warranty....got to be expensive for Ford. In a vecicle as old as yours, a misdiagnosis would cost YOU a fortune. Hope this helps.