New truck now pulling problems.

Old truck was a 2015 gmc 2500 srw pulled awesome, no problems at all at any speeds. New truck is a 2016 gmc 3500 srw now when pulling it sways all over the place. At 70 you can just feel it floating side to side never had this problem with the 2500. The only difference from the 2500 to 3500 is a added leaf spring bump stops and 18s instead of 20s.

Any thoughts on this?