Joab's Thread o' Hotshot Questions

I figured it would be rude to keep highjacking the sticky thread, so I thought I'd make my own to keep asking about what pops into my head. You guys have been very helpful so far.
So, today's questions who feel like answering...
I seem to recall reading in a thread somewhere that DOT judges the trucks weight capacity by the axle and tire ratings, not on the vehicle manufacturers given towing capacity.
Does that mean if I end up swapping out my rear axle at some point, and it has a higher rating, my truck rating can be upgraded at that point as well? What about upgrading an older trucks axles, say a 1992 F-350 DRW? Just curious on how all that works. No plans to jump into this gig with a truck older then 3 years personally. Plus the Cummings Aisin combo seems the most popular and reliable at the moment.
Speaking of which, is there any great advantage to getting a Ram 4500 instead of a 3500 out the gate?
Thanks again for all the help guys.

 

Ahh, gotcha. Thank you for the clarification.

 

Are there many folks still running trucks with manual transmissions for hotshot? I just saw today that Ram still offers a six-speed in the commercial truck line. How long are the autos lasting before needing a rebuild? Thanks folks.