On a 6% grade, the transmission should have been in 2nd gear, and at 45 MPH should be able to gain speed.
But... If something slowed you down to 35 MPH, that's too fast for 1st gear, and 2nd gear isn't making enough surplus power at that low of an RPM to really accelerate much. That's really the achilles of the old 4-speeds.... The gap was too great between 1st and 2nd, so Ford fixed that with the 5R110 in 2005.
Now, if you were already going normal highway speeds, and your speed dropped that much... I'd consider that excessive under normal conditions. Do be aware that the PCM will reduce power if the transmission is too hot, so that could have been a factor as well, but you can only monitor trans temps with an OBD device or aftermarket gauge.
