2005 boulevard c90 1500

When I'm riding at about 70+mph (85 is the fastest I've gone so far) the ride becomes bumping and shakey and not smooth at all. My friends can cruise at 90 and have an enjoyable ride. The fastest I can go and have a smooth ride is about 70mph before it becomes sketchy.

Can anyone explain what's happening and is there a way to increase front end stability without having to adjust the rear shock? The rear shock is about halfway through the threads, not near the top or bottom. I removed probably like 25lbs of rear luggage. I haven't removed this fuel tank before yet to reach the shock adjuster.

Thanks in advance for any advice and consideration!