Diesel Bombers - New to Duramax Need Trans Help?????

Ok so I just bought a real clean 2005 Duramax. It has 194xxx. Lifted 6inches with 35's. No chip or programmer, although the guy I bought it from had a PPE on in before I bought it. Had it now for a few weeks, and has ran like a champ. No problems to speak of. Just this morning I drove it to work and it ran fine. Parked it on a level service. Came out at lunch, fired right up and I put it in drive. It slams in to drive pretty hard, almost like it was parked on an incline or something. Sounded like a U joint. Then it throws a code. Drive it around a little bit, goes in to reverse but every time I put it in drive, it goes in real hard, although it shifts normal when in gear. Checked the U joints and underneath around the transmission and looks just fine. I checked the codes this evening and I get...

P0700 and P0872.:argh:

Its not acting like its in this "limp" mode I have heard so much about. But is it limp mode? Just curious as to what might be causing this and how to fix it. I don't drive it hard at all and like I said, its has nothing on it right now, and hasn't for the last 6 weeks at least. Any help would be appreciated.

BTW I am new to the Duramax. I have ran only Cummins until now. This is going to be a learning experience for me. I'll take all the information I can get.