Are you speaking of the No. 3 Frame Cross Member mounting bolts (diff drop bracket)? Not sure how the bolt mounting torque would affect your vibration unless the diff drop bracket was fabricated poorly/out of spec and deformed further under the added torque of the driveline flex.I just solved my vibration that started after AHC delete. Chased the same vibration for a month or so. Came down to SIDE MOUNTING BOLTS on the DIFFERENTIAL MOUNTING BRACKET. The FMS suggests 50ft lbs. Set mine to 42ft lbs and my 100 is back to Cadillac smooth. There’s four 18mm bolts that go through the differential mount and the frame. If they are too tight, the vibration from diff. is unable to dampen and is sent through the entire chassis. Hope this helps.