[Cat Fuzz]

I'm not familiar with the early GM 3.1L Rodeos but check the emissions sticker under the hood to see if ignition timing is adjustable. If it is it will give a value like 5 degrees BTC or something. Follow the procedure spelled out on the sticker for adjusting timing and make sure that is correct. Backfires are almost always timing related. What it is is when you have the spark plug firing too soon or too late while either an intake or exhaust valve is open.

Take it to a car wash and wash the mud away from the spark plugs and then change them paying attention to the specified plug gap. Don't assume they are gapped already. They almost never are.