I saw one of those Jonsered Iron Horse track things last weekend, I was tying to brainstorom how you could use one for fencing.
The bigger fencing dealer in the NE has one of those Wheathart's for rent. I think there's has something with pressurized water for the pilot hole. I ended up buying a Shaver because I didn't want to drive 3 to 3.5 hrs each way to rent one, plus it will get its use over the years.
I'm dissapointed with Shaver, they came out with an auger and I bought an HD-10 planning to add the auger later, then I found out from Kencove that it only goes 18". They're supposed to be working on one that does like 42", last time I checked with them it was an idea and hadn't been designed.
Jonsered Iron Horse sales brochure
http://www.maystrailequipment.com/images/iron_horse/ih_folder.pdf
1.46mb download