1996 Cadillac not shifting from 2nd gear into 3rd gear. Could it be trans speed sensor or control solenoid?

I have a 1996 Cadillac Deville that is not shifting from 2nd gear into 3rd gear. The car does great in reverse, 1st and 2nd gear but again never shifts into 3rd, it will drop into neutral if you bring the rpms up too high then back into second again when engine rpms have dropped back. Not knowing what the problem was I did a filter and fluid change and for what it is worth the fluid looked and smelled fine and the filter and pan seemed clean. This had no effect positive or negative. Anyhow I think it may be either the transmission speed sensor or the transmission control solenoid. Any chance you folks on here with more knowledge agree? If not, what do recommend? I find difficult to believe it means the transmission is completely shot being it does well in other gears.