I think from the rpms you posted it is going into high gear. Maybe skipping second and going into third. I think your engine would be running higher rpms at 55 mph. In the 3,000s depending on rear axle ratio.
About what rpm does it shift from first gear, or what speed are you traveling when it shifts?
Try shifting manually up from 1st to drive from a dead stop using moderate throttle pressure.
Here's a nifty calculator for rpm vs engine speed.
http://vexer.com/automotive-tools/speed-rpm-calculator
