I have a 2002 trailblazer. The transmission seems to be fine in 1st and 2nd, but won't shift into 3rd or 4th. Today, we pulled the pan and changed the filter. The filter was pretty clogged, but there wasn't any metal in the pan, and the fluid wasn't burned. We also looked at the torque converter through the inspection port, and everything was clean. We are stumped. Any help would be good.