2018 Ram 2500

You could do that here. Your door sticker would not change. I can go from a say 265 width single to a mid 300 width tire with the same ply, load rating etc, the wider tire would have a higher carrying capacity, up the springs if need be, and get more capacity too. One does many times have to change out the rim, deal with back spacing etc too. Nothing illegal about. I've done it, been pulled over etc, not tickets because of it. 
Many large truck firms are changing out their duals to super singles for various and sundry reasons. One ie single is about 100-150 lbs heavier weight per side of an axle than a single aluminum rimmed setup. That gives ea truck with 4 tandem dual axles another 800-1200 lbs of payload, The drivers use less HP to turn the drivers, as the tires are lighter! Better MPGs. Granted a tenth or two. 9.8 to 9.9 with a fleet of 300-500 trucks driving 100K miles a year. adds up to a few dollars. Along with the potential for fewer overload tickets per axle

This is harder to do per say with what is a completed vehicle, ie a pickup vs a cab and chassis that semi's are. It does come down to 25/35 series trucks too. You can add drop axels, heavier springs, change out the tire size etc. At the end it is up to the final body manufacture to certify the upgraded GVW of that vehicle. Think of the type C van body, Type A MH that got to the RV manufacture as a chassy only. These RV's you see with Tag axles are aftermarket add-ons from the chassis manufacture. Totally legal to do. 

I know of a topsoil, bark, rock supplier, that put drop axles on the 3 axel dump trucks. IE a steer and tandem rear. it was not so they could carry more weight per say. It lowered the number of times being stopped, weighed at scales with too much weight on the front axle. They were under their total paid for gvw, but over the amount they could have on the front axel. It happened with the rear, the front was 70-80% of the overweight tickets. 

My Navistar, I went to a slightly wider higher capacity tire, I went from a max of 19200 on the rear to 20K, front from 9600 to 12K lbs. Believe it or not, my 05 and 89 DW trucks, also had an approx max 32K I could put on those axels the way the laws read based on tire tread widths. Not that I would recommend one putting 20 on the rear, and 12 on the front axle of a 35 series truck. My Navistar is 16.5 on the rear and 8000 on the front. Me running down the road at the usually max loads of 24-25K with a paid for license of 26000, was not an issue. Original per Navistar was 18000 due to tires. 12000 rear 6000 front.  

I wouldn't be surprised if some of the above it legal in Canada too. I do know some states do not allow drop axles for additional weight capacity in the ways I've described. OR another some dump and cement trucks are using, is a drop axle that is raised off the main road but dropped so it is behind the chassis of the truck. If a typical tandem rear, single steer axel truck has 2-3 single drop axle between the tandem, and steer, along with the rear drop axle, you can add a few lbs to what you are carrying etc.

Also, lifting a vehicle can be done too. Problems here, is most states have max height rule/law/regulation for head lights. The lifting is not the problem per say, but you get red tagged pulled off the road or equal to your head lights being too high!  Too tall heavy of tires will affect how quick you can stop! You fail the field test of stopping with in x feet. Red tagged, not getting off the road without a tow, on a role deck or low boy trailer!

If one thing you think will get someone in trouble, but doesn't, there is probably a rule that will knock them off the road while part of the weight, lifting etc was the cause, it is NOT THE EFFECT of how they got taken off of the road! 

I should point out too, for those of us old enough to remember the first Ford Monster trucks, they were not lifted compared to some of todays NEW trucks, that much higher! Todays versions are not even real pickups per say.