Your traction loc is either toast and you're spinning just the one wheel when you break them loose, or you've got serious alignment issues causing the back end to "dog leg" when driving in a strait line.
You can do a make shift check by straitening the wheels, then measuring from centre to centre of the front, to back wheels on each side. If they're not exactly the same distance apart on both sides, you need to take it to an alignment shop pronto! Even a fraction of an inch can make a difference, so be as accurate as you can.