shaking during acceleration, and small thump

When I accelerate from a stop, my 1997 shakes very bad.
It is properly aligned. The shaking stops after I reach about 30-40mph, it is not constant.

Also, I have been noticing a thump that occurs after I am stopped. Usually about 3-5 seconds after I stop, I feel a small thump somewhere underfoot.

Could be your engine mounts. But it would be best to put your blazer on a ramp and check for damaged steering components.