hey dre,,, the major difference is simple really, as the price goes up so does the "race" nature of all these bikes, its all about performance. more suspension, more power, better quality hard parts is the main price difference.
with the price tag comes better parts but it also brings more maint. more service intervals, more expensive repair parts and less places to get them. thats not a bad thing if your the kind of rider who will actually use a bike like that, if your not your just turning more money into the same noise the guy on a japanese bike is.
you want something maneuverable that will easily fit in your truck, your really looking for a thumper. i wouldnt look to the drz400 or any of those because for a hardly noticeable weight difference you can get an XR or DR 650. the KLR is a good bike but i would take an XR or DR over it, they are both just more basic and simple bikes than a KLR, and talk about simple, it doesnt get much simpler than the DR650.
i could sing you the praises of my ATK605,,, but its a niche thing, like a FJ55 is in the cruiser world.
so i'll just recommend a XR600-650 or DR650 as a top contender for your search. they are both inexpensive in every way and about as reliable as the sun coming up every morning. but whatever you want to look at you really need to throw a leg over them and hopefully ride them to make the final decision as they all feel just a little different.
