Just out of curiousity since I know nothing about post drivers...don't they beat the crap out of the tractor?
To minimise any shock loads/harmonic transfer which will potential damage/fatigue the tractor, the post driver needs to firmly positioned & adjusted so the "base" is stable on the ground - follow these simple precautions, & operate the hydraulic control smoothly (not shifting valves fast open to closed..etc.) post drivers are far less stress on the tractor than backhoes, FEL's , PTO post holes diggers or implements like angled graders which create torsional stress....
The downside is correctly set up the post driver will take virually all the stress & have the "crap" beaten out of it, IMO the cheaper drivers don't hold up & prove a false economy in hard ground &/or thumping anywhere near a decent sized hard wood post- if you have a lot of fencing to do in hard ground use a commerical unit : new, s/h,hire the implement, or engage a contractor; otherwise you need to be a good welder, apt at hydraulic cylinder rebuilding & have a lot of time to make running repairs/mods on the cheap drivers.
We always use post drivers wherever possible in preference to PTO post diggers for our rural fencing- a hydraulic operation is far easier on the tractor than most PTO operations - it's also far easy & a driven post makes for a far stronger fence, though all our drivers are fitted with a reversible hydraulic augers to drill a pilot hole first.
Pictured is one of our post drivers, not cheap but the cost is easily recouped in labour/downtime saving in our farming operations (against PTO post diggers & cheap drivers) + these last a lifetime:-