I was a tax accountant in a former life. Here's the deal:
In order to qualify for the Section 179 deduction (i.e. to deduct it in a lump sum instead of over 5 years), the vehicle must have a "Gross Vehicle Weight" in excess of 6,000 pounds. That's important, as GVW actually includes the capacity of the truck in addition to the weight of the truck. My 2004 Chevy Silverado 2500HD crew cab short bed with Duramax Diesel (for example) has a GVW of 7,200 lbs.
Most full-sized pickups qualify for this deduction. In fact, may SUV's (Chevy Tahoe, Mercedes M-Class, etc) also qualify.
It is also true that you can only deduct that portion that is applicable to business use. So, if the truck costs $30,000 and you use it for 75% business use, you can deduct $22,500 in the year you bought it.
One other thing to remember: I think you are limited in the Section 179 deduction by your total revenue for the year. So, if you only had $10,000 in revenue, you are (I think) capped at $10k in what you can deduct (you still get to deduct other business expenses, so you can show a net loss from your Schedule F (farm income & expenses). Now, I'm not sure about that as it has been a while since I was in public practice....
contact a good accountant to be sure about any of this...
-E