C-20 bounces bad!

[Barnfind46]

Here is a little background on my truck. It is a '68 C-20 with 327 v-8,400 trans and corporate rear end with coils. It came with the optional overload springs. The truck was bought new in Delta Junction Alaska by a large company. It was used by them until '97 when it was brought down to the states and registered by the second owner. It had some crazy gears in the rear end like 4.56's. when I bought it the truck would shift through the gears and be in third before it was doing 20 mph.I changed out the rear end to a 2002 14 bolt with 3.73's and 30" tires. now it has much better manners on the highway.Sorry this is getting long winded.I just know the rear end gears were not stock with the automatic transmission. Now to my problem. When I hit bumps or potholes the back of the truck bounces up very hard. It jars my back sometimes. Could the company that bought it new possibly changed the rear springs out to handle some additional weight? I was just wondering if the other members had trucks that rode as harsh as mine. I drove a '68 C-10 back in high school and it rode like a dream. Before I start spending money on replacement springs I wanted some expert advice. Thanks in advance!