90% of vibration and handling problems are caused by too little air in the tires. Check the PSI once a week, as low PSI will destroy your tires, and the vibration will cause accelerated wear on suspension parts.
A steering stabilizer usually is not needed with a cruiser like this, but it's your money. Better fork spring from Progressive are a big improvement for about $70, as they perform better than the barely-adaquate stock ones.