Not a SHO, but has an AX4N, '99 Sable torque convertor locks up abruptly when in OD

[JTS03COBRA]

I realize it's not a SHO, but this car does have an AX4N trans, and the tech on this site seems quite advanced. It's '99 Sable Duratec wagon w/ a torque convertor that locks up abruptly when in OD. Just wondering if this is indicative of a failing tranny or if there's a posibility of a simpler fix...btw it's my retired Father's car he bought new-he's on a low fixed income. It does have about 170K on it...