After rereading this thread and considering whats been stated I have to think your asking the wrong question. I think what you need to ask is this "I need a new commuter vehicle and something to tow a trailer occasionally, what should I do?"
I also note a couple of pertainent questions that havent been asked, What does your wife drive/need? How old are the kids? And perhaps most importanly what's your budget?
You've stated that you don't want a truck and from the sounds of it using one as a commuter vehicle would definitely not suit your needs. You've also pointed out that you have little need for a truck, but you already have one and feel it doesn't suits your needs.
Assuming you're looking at new or very late model truck, Im going to guess your budget is 35-45k (including cash and trades), not bad to work with. My answwer then becomes find a good commuter and figure out how to come up with a decent truck. This won't be hard.
Actually I'll dial back a bit from that last sentence, I'm probably the last person you'd ever want to ask about a new commuter car. So I'll just leave this thought on that: Late model Hyundais are actually pretty decent cars, especially the larger models. Note that they are also looking to boost sales right now so a new one may not be a bad deal at all. So pick the car that best suits your needs, use the remaining budget on a truck.
Lets put some numbers to this just for the sake of the math (wether they're accurate or not can be determined later)
Trade in on the Mazda: 5000
new Hyundai: 30000
Cash outlay: 25000
That leaves 10000 or so for a truck (figuring say 5000 value of the D100) Not shabby at all. Now what to do with that:
Option 1: fix up the D100, getthe engine right, swap in the OD trans (whenthe engine is right the gear spread won't be a problem) Add sound deadening, new seats and aftermarket a/c. I dislike rear disc on trucks, especially for towing and stick shift, most disc parking brakes are pretty worthless. Hydroboost is personal preference, for me I don't care for it. It can be added easily enough if you insist. What I would do is upgrade to later 11" rear drums with bigger wheel cylinders and an adjustable proportioning valve to optimize the factory setup. It wouldn't be hard to add in the 87-93 rwabs system if you swap in a complete later rear axle. A weight distributing hitch with sway control and good trailer brakes will easily make up for the minor infraction of the shortbed. The a/c will eat up the largest part of the budget but I think you could pull this off and stay within the numbers. And yes you'll never get your money back out of the truck when you sell.
Option 2: this is probab;ly the better one for you, sell the D100 and use the 10k in cash to find a 12valve Cummins truck. Finding an excellent condition 2wdauto extended cab for less than that shouldn't pose any challenge at all. You would probably like the 94-98 trucks much better than the 89-93, i think most of those were hydroboost, pretty much all had a/c, and rwabs was standard with (i think) 4wabs optional. Note, 5 speed and 4wd add quite a bit to the ca-ching factor.
