I wanted an RC Crawler, but did not want to drop a lot of $$ on one.  So I know the stock Emaxx is already capable 4x4, and with a little modification I came up with want I wanted to start off with, without dropping a lot of cash.  

Here are the keys I think are needed to make a capable Crawler:

 (1) Flex, (2) Lockers

Speed is NOT needed

The pictures below are of the purpose built e-maxx just for slow crawling.

How it was built:

1.  Started with the stock emaxx chassis.  Modified (cut) the sides off where the batteries rested and trimmed it down, leaving only the steering servo mounting and off the rest including the right side mounting area for the the shifter servo and then trimmed off the rest even with the trani rail.

2. Factory skid plates trimmed down.

3. Stock shocks with dual rated springs (only four) placed on the inside mounting locations.  (Updated:  with the placement of the batteries over the axles, the new dual rate springs were flexi enough, however with the weight of the batteries the springs had a tendancy to not be stiff enough.  Solution:  Ace hardware, I rummaged through the springs they had in the store and found some that where stiff enough to handle the weight of the batteries, however, they were actually to tall to fit on the stock shocks without some modification.  Modifications where made to the spring retaining clips on the bottom end, meaning I shaved down the upper lip on the mounting bracket which gave more less than a 1/4 inch space, just enough so the springs would fit).  You do not have to run the typical springs, you can be creative as to what setup you prefer to run.)

4. RPM shock mounts, trimmed down the excess mounting points not being used so the batteries can fit up under the cage.

5. Locker's (Spool) were placed in both the front and rear differentials for traction.  Without this, if one wheel is off the ground the other will still grip.

6. Taller tires are a must for more ground clearance.  I used 6 inch tall tires.  With bigger tires your stock steering linkage and stock steering servor will be strained.  I used an aftermarket steering linkage for easier turning and upgrade the steering servo to rotate the massive tires.

7. Trim out the Bulkheads where your a-arms rotated for more travel up and down.  Your a-arms will flex more if you allow more travel. 

8. The Roll Cage is the key for my crawler.  This cage is the one that fits inbetween your bulkheads so a long bolt can be used.  The other cage that bolt on using your bumpers could be used, but this style is much easier to work with.  This cage was purchased off ebay and it was chrome.  I bent a solid stock piece of metal into a front bumper and tack welded it onto the front of the cage.  This is not a must, just adds to the look.  Once the chrome was roughed up I sprayed it with some paint and then cut out plastic body filler.  I used a plastic no parking sign.  Once it was cut out, I then drilled small mounting holes close to the rollcage bars.  Then I used small zip ties to attached the body panels to the cage.

This is the wide maxx stance for reference.

9.  Batteries:  For now I am still running dual batteries, and I have them sitting sideways ONTOP of the axles to place more weight over them. 

10.  72 tooth spur gear, and 11 tooth Pinion Gear (lower gear ratio) 

11.  Gorilla Single Speed conversion.  (Easier than you think to install)  Once installed trim off the Servo mounting plastic for a cleaner look.  Or you could use the servo mounting area for running dual steering servos. 

12.  A-Arms Skids:  Take the remaining pieces of the no parking sign, and trim them down to fit on the bottom side of your A-Arms, this will cover your slotted openings and keep the sharp edges of rocks out of the openings to prevent hang-ups. (Hint:  use a piece of cardboard, or paper to get your design down, then transplant that onto the sign for cutting.)  Drill small holes in the sign for attachment with small black zip ties.

UPDATE:  Given the current gearing and tire size you will need to look into a little beefier drive shafts.  3 drive shafts snapped and one twisted on latest outing. 

FUTURE MODS:  (1) Single battery, (2) Metal Shafts

I am very happy with this crawler.  From this point I can look at building the one solid axle, dual steering crawler.  But I enjoy the capability this e-maxx provides without an extreme cost.