>>1253588>Is it normal for SLRs of this vintage to vibrate quite so much when taking a picture?Yep.
Your D90 is a crop sensor camera, so right off the bat it's got a significantly smaller mirror. In addition to that, its mirror slap is motor driven and damped because that's what camera makers do with their mirrors these days.
With old mechanical SLRs like the FE2, it's not a computer gently but swiftly moving the mirror out of the way and back into place afterwards, it's usually just a spring. The FE2 apparently has a fancy flywheel inertial-dampening system to make its mirror less clunky, but it's still gonna give a significant clunk. Still, rarely enough to worry about.