Chevrolet Repair: 1992 K1500 Suburban, cat converter, exhaust flow

Question
I recently bought this suburban and do not know the vehicles history. When I am stopped at a light the RPM's begin to fluctuate way between 300 and 1000 and the suburban begins to rumble and shake slightly like it might die. It although has not died yet when stopped but it did take three times to start it tonight and I had to give it a little gas. It also smells like it is burning rich most of the time. Do you think some new plugs, wires, and a distributor would cause these problems to stop and give a more complete burn or is it something else? Any suggestions would help. Secondly, I was thinking of changing the fluids in both differentials and the transfer case. Is this a good Idea when I don't know the vehicles history and can it hurt anything or break anything loose? Thanks for your help.

Answer
Hi Charlie... changing fluids is never a bad idea when you don't know what truck has been through... as for the stall problem... a tune up never costs, it pays...make sure when you do one that you check all vacum hoses as they can cause that problem as well. install new fuel filter and some good injection cleaner when you do the tune up...also check exhaust flow to be sure your cat converter is not plugged as that can also cause a rich problem. check that 02 sensors are functioning properly as they control the mixture referance to the computer.... hope this helps... Merry Christmas