Excellent suggestion!
I forgot to account for the weight transfer due to the cord being 5.5" off the ground. When I did a free body diagram, I found that just over 2/3 of the weight is resting on the rear wheels in that configuration.
I rigged up an extension to lower the cord to about 3/4" off the ground and re-ran the test. It still barely lifts the 3 pavers, but this time 3 out of 4 wheels spin (the left front wheel does not spin in any of the tests - I suspect I have a lot of internal friction losses in that one).
I still need to re-write the code to ramp up the torque instead of going full-bore from the start of the test.