Gary, It sounds to me like a camber problem. Get a cheap carpenters level and check each wheel. If you find either wheel is off the "bubble" more than the other, it's trouble, likely a prior accident poorly fixed.
In a front end alignment a vehicle will allways pull to the POSITIVE camber side, therefore if the cross camber is over 1/2 deg its bad.
With the IRS , you will need to have thrust checked too, find the best front end man you can in your area. Its a job for a pro.
MD(mad dog)