Looking for smoother ride

I've owned my 2002 Duramax 2500 for 12 years (has only 139K miles). It's in very good condition motor & body wise, but I feel that the ride is getting pretty stiff when pulling my 5ver or not. I've never put shocks or had any other suspension updates done on it. What suggestions might I consider to improve the ride I get in my vehicle? BTW, I want to keep the costs reasonable and don't want to have to dicker around with inflating anything other than my tires.