My wife does drawn anim, and says "Are the following comments of any use?" Perhaps an extra 6 to 8 frame 'hold' on each foot after it has gone down followed by toe movement to suggest the effort of moving so heavy a vehicle, ie digging in. This may require a slightly deeper knee bend and ankle flex as the foot is still implying the maintenence of ballance. A slide can usually be corrected by placing a longer pause on the time that the foot is down. She is also inclinded to think that a little more body lurch would add to the realism of the motion. If you check your dope sheet the total time the foot is in contact with the ground should be more than one third of the whole cycle and with a heavy vehicle perhaps more. This is of course assuming that fairly fine toe movements are possible otherwise you end up with a Godzilla like thud! Hope this is of some help.