We put skid wheels on frame on a previous TT. A couple issues you need to consider (beside the bumper strength and twisting issues previously mentioned) are 1) Skid wheels reduce the already low clearance and even more importantly 2) Rear end of TT swings left/right a fair amount while turning up a driveway, skid wheels on straight axles don't. You probably already thought about #1, be I hadn't thought about #2 until I tried it and watched the wheels scraping sideways. An interesting idea that I tried once and abandoned.