Stiff sidewalls come from a variety of things. You can Have LR G tires, and have soft sidewalls. IE a Michelin XRV, designed for cushy rides for motor homes. Suck on a truck trying to carry a load per say, like my IHC Dump truck. On the other hand, staying in Michelin land, Not made anymore, but the XDE M+S traction tire I have on the rear of the IHC, with the all steel casing, sidewall and tread, they are very stiff! Do not compress with a load on them etc. A much better tire than the XRV IMHO!
On some SW GM's I have had in the past, I had both LR D and E 265-75-16 tires on them. I was usually running at 60 front, 45-60 in the rear depending upon the load. I even had the same brand and tread pattern in a D THen an E when they started being made. Generally speaking, I noticed no handling differences. EXCEPT, when I put 3 yards of topsoil in the bed, ie ~6000 lbs, or 2 yards of rock, also ~6000 lbs. A couple of times two 3600 lbs pallets of Allen Blocks! THEN and ONLY THEN, did I notice the difference between the 8 and 10 ply tire, as the one I could air up to 80 lbs, vs 65 for the other. Otherwise, save the $10 per tire, take your spouse out to dinner!
My son has 10 ply 265-70-17s on his 4wd GM 1500.....only has 35-40 lbs in them! In reality, no need for 10 ply tires, dealer had them in stock, no LR C like he wanted. Would have saved $15-20 per tire. Why?
Marty