Never been on a 73, but loads of time on 53's and 63's. This sliding/tipping thing depends on the type of ground and what's buriied in it. I've had a track lift on a sideslope before sliding, I've also slid and had a track lift when I've hit the bottom. Don't know about other types, but on firm footing, a Cat TTL will sit on quite a severe sideslope before tipping, and on loose material will, as Burnout says, slide quite easily a long, long time before the tip angle is reached. Going up a slope diagonally increases stability while allowing a fair load to be pushed, while going straight up you don't push much, or if you do you do it at a crawl. Carrying a full bucket on any amount of sideslope rather than pushing material isn't too clever, as you quickly lose stability.