Won't shift out of park

I have a 1998 GMC Sierra with 105,000 miles on it. In the morning, the transmission will shift out of park without a problem. After driving the truck for a while and then stopping, it doesn't want to shift out of park. I have to play with the shifter for a minute or two before it seems to "click" and then shifts without a problem until the next time I put it in park again. Any suggestions?