I assume it's an auto then? Slow lagging shifts and difficulty getting an auto out of park can be as simple as low fluid level. I'm sure you already know that, just reminding you. Other than that it may just be an excessivly worn tranny.

-Chris

 

I had a truck with the same problems. I just added more tranny fluid and everything slowly started to work fine. If you do have a tranny leak, look into some kind of tranny restore that eliminates leaks. It's just an additive that you put in with your tranny fluid.