1992 GMC Sierra Losing power and not reversing

This is a repeat of "Transmission question" with some added info and a donation. My truck doesn't have much power anymore. When I step on the accelerator, especially when the truck has been sitting for awhile, it revs high but gains little speed. It will still reach hwy speed eventually but it takes a while to get there. It also revs high when we get to a steep hill and to get up it I have to ease up on the accelerator and go up slowly. Early rainy mornings it may not make it up the steep hill on my driveway at all. One morning I took 5 runs at it before we made it up. It doesn't reverse well at all, if at all - if I put it in reverse and step on the accelerator it revs like it was in neutral and doesn't go anywhere. The truck had a new transmission put in in 1999. It's overdue for a transmission fluid change but the fluid isn't low and it's pink. Is it possible that this could be as simple as needing to change the transmission fluid? Or is it likely that that wouldn't help and that I'm looking at costly transmission repairs?