1987 Chevy Blazer misfires and shakes

Engine Performance problem
1987 Chevy Blazer V8 Four Wheel Drive Automatic

my truck just started acting up the other day. While driving to work I felt something was wrong when I got off the highway the truck was misfireing and realy shaking. So later that day I changed the plugs, dist. Cap and rotor, changed oil and fuel filter, I also put some water remover in the tank. After all that it still runs poor with no improvements.
i tried to check the egr valve but it seams to be a sealed unit I also checked the wires the seam to be fine.I dont know what else to check and am running out of money for parts I dont need. Please help me