1994 Buick Roadmaster Transmission slippage

My car when starting out seems OK but once it reaches 30-35 mph it wants to slip and revs high. After letting off the gas and the car warms up it will shift. The same happens when it the car reachs 40-45 mph. Could it be the bands or perhaps it is just in need of service. A friend told me that if the fluid has never been changed that sometimes it will "shock" the tranny. The previous owner told me that he had it rebuilt before he sold it to me.

The engine runs great but it has high mileage. I am currently laid off from my work but am expected to return in the next few weeks. I have a 30 mile trek to work one way.

Should I try to service the tranny or am I throwing good money after bad?

Thanks!