Slip yokes, or a sliding shaft would be an issue. It would have to be sealed completely as a little bit of dirt/mud would add too much resistance. The better design would be to have the pivot points perfectly aligned so the shaft didn't need to change it's length. CV joints have the shaft slippage inside the boot, but then the boot is subject to damage.
U-joints are very durable and less prone to damage from sticks etc........I'd build with U-joints. Unless you went way high with the horsepower, it would be easy to build the shafts yourself with limited access to machine tools.