340 -727 -3000 stall- Vibration under load - Trying to find source

Very unusual that it smooths out at higher rpms. Shift to neutral while driving and hold it at 3k instead of letting drop to idle, no shakes? Shakes in neutral @ 3k rpm in the driveway the same as when driving? If it always shakes at 1-3k rpm, see Post #9-10, or pistons replaced with aftermarket slugs of significant wt diff and not rebalanced.