1999 Chevy Tahoe water in oil

A leak down test will find a blown head gasket. If your engine is running very bad - missing - smoking - or barely running at all it is probably a head gasket. I think your tahoe is the 4.75 millionth tahoe with this problem. Very common to have the lower intake gaskets fail on these engines.
Depending how far you are going to take this repair I do suggest the following -- Based on the mileage on the engine - and the work envolved to replace the lower intake gaskets - you should remove the heads and either have a complete valve job done or just replace the heads with new ones. Brand new Chevy Performance Heads are just as much as a complete valve job - if it is done right. You can find these heads on JEGS website. Best of all they are direct bolt on and come assembled. Also, replace the distributer gear if it looks worn - it probably is. Keep us posted.