Why does my 02 dodge ram 1500 2wd will bump alot from the back if i installed new rear shocks...?

when i bought this truck the guy told me that he had lowered the back of the truck..i dont know why...so i installed new rear shocks on the back.cause he told me that was the only thing he switched in order to drop the truck from the back..i did noticed a little change of height after replacing the shocks on the back..but the bumpyness stills bad..every time i do 50mph and up it will start bumping alot i installed breand new tires also and that didnt help at all.i know its got 4 leaf springs on each side..please help any ideas???????