97 C-1500 Vibration

I noticed a bad vibration in the floor and throughout the cab, all at idle and while driving, looking at my engine and listening to it, its very quiet. My RPM at idle are rock steady, I don't think it is coming from the engine at all, could the Trans. cause a bad vibration without any noise. Its a 97 C-1500 4.3L V-6.

i would check the u-joints one might be loose or siezed. also check the transmission mount