Thanks for the input magnat. Based on your feedback, maybe I could've tried street tires with a little less grip. I think then the tires would've slid rather than hop.

For the heck of it, I picked up a $3 cartrige of bearing/chassis grease from NAPA an hour ago. I packed the diffs full. The diffs are definitely tighter, but not as tight as I hoped. I just tried it outside. It drifts good in both directions. I'll monitor it to see if the diffs loosen up over time.