Transmission trouble with '91 Chevy Lumina?

My automatic transmission has been acting strangely for a few months - going from "not so bad" to "will the tranny engage today?". Today it will not. For a while shifting from park to reverse worked pretty well, consistently - but, from park or reverse to drive did not. There was a hesistance getting drive to engage for several seconds. Then it became a few minutes to several minutes. Today, nothing at all - reverse or drive. I can hear it trying to engage when I shift to 1st, but still nothing. Tranny conditioner from the parts store was helping a little. The fluid's good and topped off. There are 70K original miles on the engine/transmission. Granny miles. Driven daily, but for short distances (4-12 miles at a time), and only on the freeway 3 times/week.

I have a feeling it's a linkage problem because I was able to coax it into gear by gently rocking the shifter between nuetral/rev./drive with some success for a while. However, I know a 20 yr old car may have some tranny seal issues as well. Dried up? Btw: no leaks, ever.

Any ideas would be appreciated. Thx.