I still don't beleive the rear leaf springs on the D100 are so different as to account for 1000 lbs less payload than the D150s. And, the D100s had less std. equipment overall than the D150s.
My shortbed '85 D100 (almost done swapping the orig slant 6/904 with a low mileage 440/727 drivetrain from a '77 Dodge Tradesman 2 1/2 T chassis (motorhome)) has the optional 35 gallon tank, A/C, PS, PB and 'Prospector' I pkg., so it is effectively equipped like most D150s. I just have not seen any obvious differences in the leaf springs between the D100s and D150s ... in the parts catalogs or at the salvage yards.
This subject (D100 vs. D150) is really of interest to me because I am about to cure my power problems with my engine conversion. Incidentally, my totally stock front suspension sits about 3/8" lower with the 440/727 installed than the orig. /6 & 904 (about 140 lbs. difference).
Since my project will evolve, I started with front suspension upgrades including pulling some parts from same/similar year Ramchargers incl. the (very large) front sway bar. It is the rear suspension I have the most questions about - the rear leaf springs and the payload capacity for the D100 vs. the D150 ... 'cause everything I am seeing on the rear suspension under my D100 appears to be the same as the D150s, yet the original literature explains that there was this huge 1000 lb. capacity gap between the D100 vs. the D150. The rear axle and brakes appear to be identical as well. If anyone has any experience suggestions on this subject, let me know. My 'new' 440/727 drivetrain is a stock (at this point) smogger motor from '77 w/ the HD 727, so I am after the torque increase of a large cube V-8 - nothing crazy (yet). I would like, however, to work toward beefing-up the suspension to handle towing a car & trailer when I need (& without worry).