New LCA's-getting strange vibrations

[bark3rd]

I have installed a set of HPM Megabite JR Lower control arms and have some vibrations and rattles that concern me. The most noticable vibrations occur when I am at high speeds, over 90 mph and let off the throttle, when I do that the sound/vibration is similiar to the sensation you get when driving over some bridges (sort of a humming). For some reason I also seem to hear the differential clunking/engaging when I am shifting into 1st or 2nd at low rpms. What gives? The mechanic says I am to expect things like this with aftermarket LCA's but things sure seem more noisy. I thought these things were supoosed to add rigidity, etc. The megabites come with instructions for 2 different setings, one for full drag and another (lower) setting for all around use-I have it on the lower setting because the mechanic said they would bind up the rear end set at the drag position. At least I can say that my 60 ft times are consistently 2 10ths lower since the installation.

------------------
Black 2000 GT,5 spd,17" wheels,4:10's, HPM Megabite Jr LCA's, Flowmasters, Steeda pulleys, timing adjuster (11 deg), K&N filter, Nitrous Express w/purge (75shot currently),NGK spark plugs (lower heat range than stock plugs), airbox silencer removed