It turned out to be worn-out/sprung window regulators. Put in new ones from LMC at 39.00 each and they go up and down easier than I thought possible. Now nice and tight, no rattles and easy up - easy down!

40 chevy 1.5 ton, 50 chevy 3/4 ton, 67 chevy stepside 1/2 ton, 67 C50, 68 C50, 73 chevy 4x4 3/4 ton, 2004 Silverado 1/2 ton , 2005 HD 3/4 4x4 Duramax (don't even get me started about my Oldsmobiles!)