Mainly flat cruising. This is a problem that we definitely did not think would happen when we purchased the suburban, as we purchased it specifically for towing.

 

Double or single axle trailer? Maybe try playing with the tongue weight?
I used to tow a double axle 4,000 lb boat with both our '93 and '05 Sub; both pulled fine.
Do you have a dealer you frequent with your boat? Maybe thay would have a suggestion.

 

Double axle trailer.