Lowering only rear ride height will make the problem worse (decreased suspension travel when loading up, or worse, ground contact at the rear under acceleration), but increasing rear toe is a very good call.
Here are the steps to take in order of effectiveness...
Softer rear tires or foam tires all around (a good set of foam tires on a good surface will last as long as rubber due to less wheelspin). Don't be tempted to use softer foam (lower 'shore' rating) in the rear, this will just make them wear faster. Use the same or slightly harder shore on the rear. Also use the wider foam tires on the rear if you have clearance (28 or 30mm).
Lift the car off the ground, turn ONE of the rear wheels. The opposite wheel should turn in the opposite direction with little resistance. If it appears to be locked (both wheels turn in the same direction), then this is no doubt the problem, look at the rear diff to determine what the problem could be. If using ball diffs, loosen them, if using gear diffs, check the internal gears or oils used inside.
Increase rear toe-in, it is normal to have as much as 3 degrees of toe in at the rear, but for a touring car, I would suggest starting at 2 degrees on each wheel and work from there. If the wheels are pretty much in line (parallel) then this is what is causing the problem.
Increase rear droop. Droop is most easily translated as the amount by which you can lift the rear of the car WITHOUT the rear wheels leaving the ground. Think of it as negative suspension travel. The bumpier the surface, the more of this you need. It is normally adjusted using grubscrews in the rear arms which contact the chassis, shortening suspension extension. Start by taking the screws out, or making sure they do not contact the chassis (full droop), and go from there. The right amount of droop is a tricky setting to get, but for stability you want as much as you can get.
Raise the ride height a little at the rear for more squat absorption. If the car is lower at the rear, then try making it level by only raising the rear a little.
Use softer springs in the rear shocks, or softer oil, or an extra hole in the rear shock pistons.
Put a stiffer anti roll bar on the front or stiffen the existing one (move the silver retainers along the ARB toward the center of the car). Anti roll bars affect traction at the OPPOSITE end of the car. A stiffer front gives more rear grip, a stiffer rear gives more front grip. Don't go too stiff though, this can have a dramatically detrimental effect on the handling.
Reduce your toe out at the front a little. Toe out is necessary at the front to get good turn-in, but more toe out makes the car turn more aggressively, encouraging the rear to step out.
Lastly, try to avoid wheelspin.
Hope that helps. Once again let me stress that droop is very important to understand and can make a HUGE difference to rear traction, it is the most overlooked setting in my experience.