Ya, excuuuuse me for trying to make sense out of your post, YOU'RE the one who listed your rpm's at 5000-6000, not ME. Then just a big run-around when I tried to correct you in a nice way.
If the flex disc and coupler are in good shape, you can also look for other signs of drivetrain lash, such as the center joint and mid-ship bearing, and excess lash in the differential, this all has to be checked with the car in the air (ie on a hoist or the rear wheels jacked up off the ground).
ps we have the weird timing thing going on again, this should be behind SteveB's last post. The time on this post is correct, I posted it at 741am. Steve, I assume your time zone is set correctly (Eastern time)?