Chevrolet Repair: 88 GMC 4x4 problems, gmc 4x4, cat converter

Question
I own a 1988 GMC gas 350 4x4 3/4 ton truck with automatic trans.I have 2 questions. First if I accelerate quickly I hear a sudden rattle about the time the truck would shift and the check engine light comes on and I seem to have lost power.
Second question, the 4x4 has always worked, I would pull the lever into 4x4 hi and a few seconds later the light would come on and the front drive would be on. Now it seems to ingage when I pull the lever into 4x4 but the light does not come on and the 4x4 doesn't ingage. Where do I start looking? Thanks Mel

Answer
Hi Mel...loss of power could be a few differant things...first check for a plugged cat converter....then i would check fuel system, plugged filter bad inj etc... 4x4 could also be a number of things....check vacum to front actuator... check vacum electric port that controls front actuator. check fuse that controls that vacum switch... check cable at vacum actuator ... hope this helps