blowngmc

I have a 2006 Gmc 4x4 Quad Cab 5.3l and a maggi. Trans was rebuilt with all the goodies about 15k ago. I pull a heavy boat and recently have been hearing a grinding noise when I decelerate on the highway. No noise while accelerating and no slipping. The other day at the lake went to put it in reverse and it makes a grinding noise the whole time its in reverse. I crawled underneath it and it sounded like coming from trans but you know how noises travel. Put the boat in and parked it. Come back from fishing and put truck in reverse and just grinds and wont go into reverse. Sounds like your trying to put a manual trans in gear with out the clutch. I gave it a little gas and just grinds more and no movement at all. I finally pushed the auto 4x4 button and it stops grinding and goes into reverse and no other problems. Left it in auto all the way home and didnt hear any grinding on deceleration like before. Put the boat away put it back into 2 wheel high and still no grinding or problems! WTF? Is it the transfercase? im thinking because if it was the trans it should still not move if i mess with the auto button right? I have changed the t/case fluid about 8k ago and did use the required gm blue special fluid at that time. the truck has less than 50k miles on it and have blown the trans and the rear end and have expected the t/case to go next but now its not doing it. Any ideas? it is the Z71 model. Any one have similar problems? Also a note that no check engine light or trans or 4x4 light lit on the dash