http://www.autosite.com/content/own/...w/seriesid/588
Your problems are all too similar to the recall GM had on the earlier 97-03 generation GPs. However, according to the site the 99 isn't included under the recall but I would still check with GM/your dealer regarding the matter.
The above link is for my 97 GP which we took into the dealership and they fixed it per the recall. You can tell if it's been done on the car because they take a knife to the gray steering column boot and split it open to access the steering knuckle I believe.