Weird problems need help
 

Hey guys, I hope you guys can help me out I'm having a couple problems but to start I have a 2007 Silverado LTZ 4x4 with a 7.5 inch Rough Country lift on it. I don't know to much about it cause that's how I bought the truck. 1st issue - when I put the truck in reverse and if I hit the gas to hard it feels like it disengages and a loud grinding noise and truck won't go any where.( grinding noise as in its spinning and scrapping until it slows down) truck isn't moving. I put it in drive and reverse and it's fine if I coast out but if I hit the gas I wanna say it does it at like 30% pressure of gas pedal. 2nd every once in awhile from a dead stop I start to drive and my speedometer will drop to zero and truck will not shift out of first gear but when I come to a complete stop then it works. 3rd my cel just came on and I got P0449 so emissions code what's the common fix for this. The first 2 issues do not throw any codes or anything and they got me stomped hope you guys can help me out thanks in advance