99 LHS stuck in limp mode?

My 99 3.5L LHS will not shift. It seems to be in 2nd gear, will not shift. It makes no mechanical noises at all, will not respond when gear selector is in 3 or L. The speedometer works. My research has led me to believe it is the output sensor. Anyone know where it is located? Should I replace the input sensor too, being that they are only $17 each? How do I determine which sensor is input and output?
Thanks!

I fixed this same issue last year, but I assume you have fixed this by now?