Originally Posted by SupremeSteak1
It certainly sounds like a mechanical issue.
Well, it is always a mechanical or electrical issue... : )
Seriously though, Like was mentioned earlier in the post, it is very likely that the problem is that one side just has a higher resistance.
Alternatively, if your drivetrain is scraping the ground, or if you have bolts, nuts, etc. poking through the drivetrain and scrapping the ground that could case the drift.
Lastly, often times (from what I understand) motors require more current to spin 1 direction than they do another. So as your drivetrain may have 2 sets of motors facing different directions, that could also be the problem.