Any Ideas what might be causing this, and how to fix? Also, Thermostat Replacement requires removing IM? 67000 miles. V6 DOHC 4x4.

Sounds like a blown head gasket, you could try aa compression check if you wanted to be sure.

This is most likely a cracked cylinder head. Supposedly, 1998-1999 3.2/3.5's had a bad run of heads. The tell-tale sign is coolant in a plug well.

G/luck
Joel

Something I hope I never see. Good luck getting it back in shape!