1995 Chevy Silverado Won't get out of first gear when warme

I drove my 7.4 L. One ton Chevy 3 speed w/overdrive to Dallas last Saturday to see a movie. A 60 mile trip. The truck did great going down there, it ran about 2500 rpm's at 70 mph's in O/Drive, but when I got off the interstate, she dropped down to first gear and stayed there until I parked. After the movie, she ran great for about 10 miles, dropped down to first gear at a light, and stayed there all the way home. I did'nt go over 50 mph's at around 3200 rpm's, but when I let off the throttle to coast, it was like she went out of gear and idled at 1200 rpms. It normally idles at 800 to 900 rpm's in park. I have been driving around town for the last 2 days and nothing has happened. Could it be a clogged screen? Maybe a linkage problem? Maybe a speed sensor? Or am I just screwed? Please help me with some info. Thanks.