94' Sable shifts funny

I have a 1994 Mercury Sable with a 3.8 Litre V6 engine. Automatic AXOD transmission. It shifts out great for a while, then it will shift into second and won't go any further. Before anyone asks, I already changed the fluid and filter. Any ideas?

I am unsure what the problem is, but my 93' 3.8 surges at highway speeds. While driving at about 70 MPH, I can feel the vehicle surge forward then release, while not seeing any drastic increase in engine RPM on the tach. I also notice that, while driving at highway speeds, if I let off of the gas pedal and the rpm's drop to 2500, when I step on the gas pedal again, the engine winds up to about 3000 rpm before I can feel the trans engane again. It feels as though the vehicle temporarily drops into neutral. Do you have the same experience??

1st check fluid?
ok, then change filter?

if either doesn't fix it, it's going bad