I am sure people already answered, but since I owned a crawler and my dad the trekker, and through countless mods i found a few differences.
when it comes down to it the only parts that are the same are:
-axles
-shocks
-tranny
-servo, and ESC/reciever
-motor
diffrence is:
-the parallel metal plates are completly diffrent
-The crawler is SHORTER, so the drive shafts are noticeably shorter.
-The stock crawler battery is a 180mah, the trekker has a 220 battery.
-and the crawler has softer more rounded crawling tires, trekker is a little harder and square. probably more due to shape than the compound. but the trekkers looks and feels more real, where the crawlers is better on the rocks
my crawler didnt come with extra gears, where my dad's had a couple extra sets.
overall the trekker is basically a larger better crawler. If I bought another one I would get a trekker every time, and maybe buy the crawler's tires separate.
Wheelbase side the trekker is closer to 1:20 where the trekker is 1:24 (when looking at hobby grade static models for body's this was true)
If you plan on modifying it with anything like the blade SMC chassis get the trekker, its easier to take away plastic on the drive shafts and such than it is to stretch them. and honestly the parallel plates of the trekker are more durable than the ones on the crawler.
Oh, and on another note, the Crawler HAS been discontinued as far as I knew, the trekker replaced it, but people still carry the crawler and they carry the parts is many stores still!
here's my build thread, i have quite a few pictures with a ford truck on the SMC chassis. it was a micro crawler at first but I got most of the trekker parts for the larger chassis at one point. and the final version (purple jeep) is a whole new monster of modification to get it to work.
http://www.rccrawler.com/forum/losi-micro-rock-crawler/401283-%5Bbuild-thread%5D-chevy-ford-finaly-jeep-warning-lots-images.html