Safety checks in Ontario follow certain criteria, like working lights, brakes, certain amount of tread left on the tires, no holes into the passenger cabin, exhaust must end past the passenger cabin, working seat belts (if the vehicle was originally equipped with them), windshield not cracked (in the drivers view or wiper path or something like that), your horn has to work, steering linkage cannot have more than a small amount of play, and a couple others I can't think of at the moment.
Most mechanics are good judges of overall mechanical fitness of vehicles and usually after meeting the criteria the vehicle must meet will pass the safety inspection if modifications are done well and look sturdy. (this has always been my experience)
Some people, mechanics not excluded, take it upon themselves to crusade against anything they feel improper, even if it meets the norms set forth by a governing body. Maybe this is what you are dealing with?
Ask around for shops that deal with modified 4x4's regularly and check there if they would safety check the vehicle. It may be that the mechanic you are dealing with is not experienced with modified vehicles and therefore not comfortable certifying it.
Also, are you in the City? Try a smaller shop outside of the City, chaces are they are more experienced with modified trucks.
If they know you are getting it safetied for yourself, not to sell it certified, sometimes makes them feel safer and pass your vehicle. Also, safeties are valid for 30 days only, if you know the shop, promising you wont drive it on the road for 30 days, or licansing it on the 29th day may elliviate their concerns for getting stung, should you have an accident and it comes back onto them.