If it was just the lighter springs, then adding a airlift/Timbren to adjust spring rate when loaded would solve that problem (although you'd still have the "official" GVW issue. Bottom line, im now leaning towards the colorado Z71 in some configuration or other. Or I could just hang onto the Ram 2500!
With how you use the truck, I doubt you will ever use the front locker other than to see that they work so I would not worry about that too much. I am not really up on the Colorado, but a quick look appears to be about a $6k difference between the z71 and the zr2. $6k goes a long way in aftermarket suspension and other mods.
Besides, if you sell the Dodge, I can swap my well-worn shocks for your nice fresh ones .
By the way, just contacted by a Chevy Product Specialist (after a fair bit of trying...) Word is that the payload reduction is due to the special shocks on the ZR2 and the fact that they cant be run in an over-compressed mode. Thus, so I am told, if you add airbags or suplemental suspension to lift it back to "normal" position when loaded, it should be ok with full payload. Also, they say there should be no issue resting the motorcycle tire on the rear gate and that it is designed to hold 200lbs when down. (There is that whole "bumpy road" thing but??)
Impressed with this guy as he seemed to have done his homework. Now, I've asked for documentation on this to protect warranty/insurance/liability, etc. Not sure if i can get that, but I've asked. Moving to the Z71 does seem to solve all issues so there's that alternative. Of course, then there's the "no sticker" problem and my loss of "trail-cred" with the other guys. Once resolved, we move on to the ever popular "long bed/short bed" debate - and then the always interesting "which oil" and "best tires". (Can you say "anal retentive")?
(Raises hand) ZR2 diesel owner here folks!
So the official reasons for the lighter towing and capacity on the ZR2 is the lack of the air dam helping to cool the truck at lower speeds and the DSSV shocks/spring configuration. HOWEVER! That being said, the rear and front axel housings, shafts, control arms, and other suspension components are beefier than the other Colorado trim lines and made to take a beating offroad that would snap a z71's front arm in two. I would say that the carrying capacity of course is a suggestion by the manufacture for you to get peak performance out of the ZR2 truck with the new suspension set up when off road. More of a cover your own ass move than anything else.
This is pretty evident from the spec sheet on Chevys website that shows the zr2, regardless of extended cab vs crew cab or gas vs diesel (with 8 spd and 6 spd tranny respectively) carrying capacity and towing capacity to be the same, even though the set ups can range from 86 lb to 106 lb difference between them in favor of the extended cab with the 6ft bed being lighter, AND despite the fact that the diesel out performs the gas by 700 lbs on towing on the standard trim models. Note: exact same 8 spd transmissions are paired to the gas ZR2 as a Z71 and 6 spd tranny to a diesel ZR2 and a Z71.
So numbers break down of the ZR2 diesel crew cab (just for example since that's what I drive) show a payload of 1100 lbs (zr2) vice 1513 lbs for standard trim, and towing of 5000 lbs (zr2) vice 7700 lbs standard trim. Now could I probably trailer 7700lbs with my ZR2 diesel if it was a balanced load with perfect tounge weight and I didn't drive like a mad man down the highway? Probably. Would I do it for any great distance or time? Probably not. Do I think I'm going to break my truck if I toss 413 more lbs of crap in the bed than its officially rated for when I drive down the highway, of course not. I also don't expect the same performance out of the truck when I go to take it off-road with those kinds of loads as well. Weight will always be a determining factor anytime you tackle obstacles or mud or sand or any other terrain . . . . along with tires, airing down and total ground clearance.
As to the tailgate issue, you could always get the extended cab vice the crew cab and gain the 6ft bed. But you lose seating for the rear, so it would be a give and take. I don't think the tailgate would be any different from the tail gate on the standard Colorado's since they use the same one. Part numbers match between the various models. All of them have the check cables to support it when its down. If your able to close the gate with the bike at an angle and snug it up against the wheel, I wouldn't be concerned about it. Just ensure your strapping the bike down properly (which Im sure you do) and there should be no problem.
Hope this helps a little and if you do get the ZR2 let me know, always glad to see another one on the road.