Even if your truck weighed 10,000 lbs which is probably on the high side you have lots of safety assuming the ice is consistent thickness. If you use the calculations on this website,
http://lakeice.squarespace.com/bearing-strength/ which I have found is on the safer side of others, and use Talking Moose's 20" as a minimum I get a calculated 20,482lbs the ice is capable of handling at 90% efficiency. More than double my guess of your truck weight. Even at 60% (I wouldn't actually go on at this percentage) the ice is still capable of handling 13,654lbs. I am not saying you should drive on because I am saying its safe. If I had a truck like yours I would still drive on but I would of keep clear of the lake inlets, outlets, pinch points and not speed. But that's me. You do as you feel safe to.
If they can run 40 ton semi's on the ice roads up north on 40" of ice I don't see how 20"+ won't hold your 5 tons of truck.