Hi Mav, welcome to the forum!
What do you mean by wobble? My '09 STX sits solidly on the road, but then I don't accelerate hard or brake hard. I tow a bit (well, that's an understatement of sorts) and the entire rig sits nice and tight.
Are you talking about a shaking or shuddering while cruising? Are you talking about a yawing motion while taking off in dirt with the wheels spinning, or do you mean when you straighten the car up after a turn, the car bounces shifting its weight from the left side to the right?
The rear springs of the D40s are a bit underrated, and the shocks aren't the greatest. If you mean the weight shifting left to right and back again, that'll be shocks.
Otherwise you could have a diff issue (some Thai trucks had the wrong oil put in their diff), or you could have some other issue with the drive train. Depends on precisely what's happening.