1996 Chevy Tahoe Engine seems to be missing.

Hello,
I'm pretty good at figuring simple things out on my truck, however, not this time. My truck was running great. The truck was shut off for a few hours, and ran well prior to shutting it down. When I started it back up the engine had a heavy vibration and seemed to be misfiring or something. When put into gear the truck stuttered and lagged. In park when given fuel it sputtered up to a higher rpm. I was able to drive it home but it was a shakey ride. I ended up changing the distributer, spark plugs and wires, fuel filter, air filter, and drilled holes in my catalytic converters to eliminate any of those possibilities. It still struggles. So my questin is what next? Would it be a sensor issue, like the knock sensor causing the timing to not self adjust? Just wondering if this is common, or should I just swap out the engine? Thanks for your time.