I have a assortment of 4"x 6" and 5"x 5" barn poles and RR ties cut up in 2' and 3' lengths for blocking up any vehicle on my place.
My 11200 lb 5er is setting on four stacks right now with the wheels pulled waiting for me to mount its new brake shoes. Each rr tie stack is at different angles to eliminate them from rolling.
I use 4 jacks like the pic when I rotate tires or do brake jobs but if I have to go under the truck I add my cribbing at each end.
Concrete blocks are safe if used on a hard flat surface like mobile homes are set up on or our older homes use. But I sure wouldn't put a lot of weight on one thats sitting on just the ground.