2004 Chevrolet Trailblazer Electrical Problems

First thing, have a shop (or do it yourself) use a separate oil pressure gauge (not the one on the dashboard) to verify engine oil pressure. This is to avoid the possibility of any engine damage.

Next, have Autozone or somewhere like that to scan your computer for codes. Post the codes here for more assistance. (This will likely steer us in the right direction to address the misfire problem you're having.

The stuck speedometer sounds like a steeper motor on the rear of the speedometer going bad. Let's deal with the oil pressure and engine misfire and then work on the speedometer (if it is still a problem).