I had a 454 4WD 1977 Chevy C30 (one ton) with a precision sprayer and a PTO powered Defco pump.  I had custom overload springs built to carry the 550 gallon stainless steel tank after I found the PTO shaft rubs on the differential when fully loaded.  Those overloads solved lots of problems, but also hint of the heavy duty required to operate such a sprayer.  I loved the speed I could cover ground, but be aware it is very hard duty for any pickup.  I had to carry the small shotgun type filters that go in the fuel line next to the carb, as when in the field, I sucked so much gasoline through that thing even a small amount of residue would cause missing and sputtering.  Engine would run fine on the road back for a fill-up, but sputter while spraying.  I finally figured that was a problem, and changed filters quickly after engine began running rough.  After learning that, I had few problems.  I would cut open the paper filters, couldn't find anything, but knew it was there, as changing filters solved the problem.

That paragraph above explains what will happen to fuel use in a gasoline engine truck.  I was short on labor, finally decided it was cheaper at $3.00 per acre, to have spraying done by a custom operator.  If I had plenty of time on my hands during planting, I might have made wages after expenses, but that was before post herbicides were used.  Now, with posts such as Roundup being popular, your time use has changed, so it might just be an economical move for you to mount the sprayer on a pickup.  You do save a lot of time with a pickup over a tractor spraying, and I loved doing it.

Now that diesel engines in trucks are more refined, I'll bet they would be much better, with that improved torque.  I maintained the same RPM in low 4WD that it took to make the truck go 55 on the road in high. (it was full-time 4WD)  Those RPM's when spraying often made me put the petal to the floor on hills and soft ground.  I had to maintain RPM to get consistant volume with the Defco metering pump, and I bet a diesel, with higher torque rise, would have been much easier and use less fuel.

Lots of luck with whatever you decide.  I liked spraying with a truck, just pointing out it does take a lot of extra fuel and is hard on transmissions and engines.  Most of the custom operators in the day traded trucks often, or sprayed a couple of years and used it for a sales truck after putting a new truck under the sprayer.