I just bought a VF Dig setup for my comp rig. With the VF Dig setup, you need a HiTec 225 micro servo, and everything else is included. The VF setup installs directly onto the axial transmission. I like the VF setup a lot, but because they are viewed by many to be the best dig option for the axial, they are VERY hard to find. When they were building them and selling them, they were always out of stock at most of the crawler stores. The company recently sold out, and I don't think they're being produced right now. The guy that built the VF parts is still on one of the big crawler forums, and support is going to be around for a while for them. Some other options are the RC4WD R2 Disconnect tranny that's an all in one unit, and the CKRC FT Dig, that is manufactured by RC4WD for CKRC. I just bought my dig, but I will be doing a how to article on installing and properly setting up the VF unit.
VF Dig Systemhttp://www.visionaryfabrication.com/sales/vfaxialax10digsystem.htm
Assembly Instructions (VERY good educational resource)
http://www.visionaryfabrication.com/sales/digbuild.htm
There are also driveline digs that can be installed basically in the rear driveshaft, after it exits the tranny. However, everyone that I've talked to considers the tranny options to be much more durable and reliable, and a lot less of a headache. The driveline digs are usually built for trucks that don't have a readily available dig option like the axial does.
Another thing that you're going to need is a three position third channel on your radio system. If you want to stick with a pistol grip, you can either modify a Traxxas TQ3, or use a Spektrum DX3R or Nomadio or Multiplex controller. My problem has been that I have 2 very nice radio systems, but neither was 3 position third channel. So, I had to wait until I could save up to buy a Spektrum DX3R. I don't know if the Futaba 4PK has a channel that will work or not. I'm hoping it does, because I really like Futaba a lot. I have a 3PK and 3PM, and wouldn't buy anything else if I could help it. The TQ3 modification is an easy mod, and is the inexpensive option. You can find the systems already modded on the forum for $25, but they go quick. You can also buy a new one that's been modded for about $60 from either CKRC or TCS, but I'm thinking it's CKRC. Hope this helps some. I'll be starting my comp build with the dig setup next week once the parts have arrived.