Ever since I had all that work done on my 4WD, (see previous post) my truck has been running like a dream. Until yesterday, when I was sitting at a stop light...foot on the brake...shift lever in the drive position. After about 30 seconds or so at the light, my truck kicked out of drive and into neutral. (I know this because I heard and felt a normal shifting clunk and when I let off of the brake, I started to roll backward). I looked at the shift indicator and the square block behind the D was flashing. The truck would not move forward at all. I quickly shifted it into 3 and the transmission engaged. As I was going down the road I re-engaged the indicator into D and all seemed well. When I got to my destination,I tried to simulate the conditions (sat in the parking lot with my foot on the brake and in drive) and all seemed fine. Tranny temp was about 150 and no malfunctions. The last 24 hrs have been OK also.
Any Ideas on this problem...or just a Fluke deal?

Sounds like an NSBU... A search will yield more reading than you can shake a stick at: http://dieselplace.com/forum/search.php?searchid=597482

You should fill out your signature so we know what kind of truck you are driving/talking about.

Moving to Allison OEM forum.

Oh yeah! 2002 Chevy 2500 HD Duramax. Long Bed, Crew Cab, 110,000 miles.:D

Bad NSBU or connection at the T-case

Yeah, I'd check to see if you have the black or tan NSBU switch. If it's black, change it.

Did it set any codes?