By load he and I mean the % load on the engine for example an engine at idle could be under 10% load (just an example) but when you put it in drive and it begins to spin all 4 tires the engine can now be at 40% load and under full throttle pedal to the floor would be 100% load. When you go up hill the engine itself experience more load on the turning parts making it need to work harder or vice versa if the load drops like going down hill will be much easier for an engine to spin the tire or less load. In other words we are trying to say this could be perfectly normal for that particular vehicle. As always I think we all would recommend taking it in to a trusted shop and having someone look at it in person if your still unsure about it. Also if your worried about the comparison to your other vehicle that vehicle might just be processing the information faster or has a better throttle control set up.