My opinion is that OP is looking for more than just do the job. His Durango is just doing the job, but he is willing to replace a less than 2 year old vehicle with something else. My opinion is that it should be noticeably better.
The Nissan 5-spd trans gearing with 2.94 axle is very similar to the Durango 545RFE with 3.92. The Nissan 5.6L is down a lot of horsepower compared to 5.7L Hemi, but similar torque.
The Ford and GM 6-spd transmissions offer a much shorter first gear than both of those, with 38% more wheel torque with 3.73 axle plus at least 370lb-ft of torque from 1750-5000rpm from the Ecoboost.
You'll have to compare pricing, but leftover 2014 Expy should be had at about $10k off MSRP. I have not priced a 2015. Also, check out the GMC Yukon Denali. I do not know where it stands on payload compared to your Durango, but the engine/trans are strong.