How do i stop my 2002 GMC Safari from swaying

If this excessive swaying is due to worn suspension then those worn parts will be tie-rods, shock struts, anti roll bars and the rubber bushings holding the various parts of the suspension in place. General suspension overhaul would be required.
Now if its a normal feature of the vehicle you can improve the handling by fitting harder anti roll bars(sway bars), stiffer shocks, custom tie rods, better quality rubber bushes (polyurethane) and by lowering the suspension with shorter stiffer springs.
I suggest searching for an owners club for your vehicle where you should find some people who are doing the same thing and get advice from those who have been there before - --- You can ruin a cars handling and make it quite dangerous by changing things around, I suggest you search for suspension modification and do a quick 101 of the do's and dont's of suspension modification before you start swapping parts.