I'll launch the first reply to your post, biggman100...The heavy duty shocks, unless coil spring boosted, will not help ride height. For either truck, the first consideration is a load distribution hitch and appropriate hardware. (See the exchanges with JoeMac51 at our trailering forums.) If you cannot readily control ride height with a load distribution/equalizing hitch, then rear spring rate is next.
I'm not a fan of air bag boosters, though I encourage others to defend their success with this approach. My visual on leaking or blown out bags is not a pretty picture: The sudden impact on vehicle handling, including loss of vehicle control with a severe load wavering across several lanes of traffic.
If a set of moderate, properly designed helper springs will not do the trick, custom springs with added leafs might. Here, however, we're approaching the limits of chassis/frame design, powertrain limits, braking limitations and the vehicle's original intent.
The Ram 2500, if originally a factory "Heavy-Duty" package, should handle this load with a moderate spring rate increase to no more than the level of the 3500. Frankly, I'm more comfortable with a bona fide 3500 chassis, though both the 2005 2500 and 3500 are essentially the same if Cummins equipped or a factory "Heavy-Duty" package. The 3500 has the dually option, which is a good idea for hauling equipment. As an ex-heavy equipment operator, I'm fully aware of off-highway construction equipment weight, even for "lighter", owner/operator equipment like a Case 580 with backhoe. This is dual real wheel towing territory.
For the Dakota, again consider the chassis, axle and powertrain limitations. This truck does have a 3.9L V-6 and AX15 transmission as your barometer. About the time the load taxes the rear springs dramatically, you've also overloaded the 3.9L V-6, AX15, the axles and brakes. That's why we have various tiers of chassis GVWR. This means something: Like time to look for a heavier duty truck if necessary.
When wife Donna and I stood on a local Dodge/Ram lot in October of 2004 and looked at 50 new 2005 4x4 units, mostly 2500 and 3500 models, I singled out the SWR Quad-Cab 3500 4WD with the short box and 140.5" wheelbase. Why? I knew it would handle well with the lesser wheelbase and that we tow, seldom carrying anything in the bed.
Was the 3500 overkill for our intended use? The Cummins diesel would deliver better fuel efficiency than any of our other light trucks, including the 1/2-ton and 3/4-ton varieties, our Suburban 4x4s and the Jeep 4x4s and SUVs. I knew the longevity of this truck was assured...In our case, we ran the original front brake pads to 105K miles, and they still had 50% material left when I changed them—typical for a 1-ton capacity chassis with our kind of highway and periodic towing, using trailers that have well-maintained and adequate brakes!
On that fall day, when the salesman asked whether the heavier springs were necessary, I shared that they were the least of our concerns—quite the opposite, if the ride quality was not unbearable, we planned to drive that 3500 empty 85% of the time...Turned out the ride quality was outstanding and no different than a comparable, single rear drive 2500. The only time the 3500 spring rate aspect comes into play is when there's a heavy load on the truck's back end like you're describing, biggman100!