I have a few ThinkPads that have the rock when the lid is open and when moving the laptop the LCD lid would rock back and forth, this problem is not just for ThinkPads but even HP and other brands. My 600X, T40, R50 and X40 are still solid, but for my i series, R31, R40 and some others they rock. Some hinges have screws to tighten but some dont. Most if not all HPs I got in did the rock with their LCD lids.
IBM: 700C,701C,760XD,755CD,770Z,360Cs,600X,560X,560Z,570,310ED,380Z,390X, i1200,i1400,240,850,A22m,A22e,A30,G40, R31,R40,R50,R60,R61i,S30,T20,T23,T30,T40,T60,T61,X21,X30,X41,X41T,X60,X60T,Z60m,TNote,X3200 Server, NetVista M41 6792,M41 6790,X40 Aptiva 2170,ThinkCentre S50,S50 Ultra,A50p,M50,M51,M55e,WorkPad 20X,Z50, ThinkPad T420