'03 Denali Traction control & downshift problems! HELP!

Hi Everyone, I have a 2003 GMC Yukon Denali and My check engline light has come on and it says on the little computer screen something like "Down Engine Power", and "Stablizing system disabled" and it really won't go... I can get it to go up to about 30 mph, but not much after that....??? Can anyone help me figure out whats wrong? I really depend on my car and need to get it fixed soon! Thanks a bunch!