So here goes- 99 3500 van with 5.9- 94 long block, with all stuff switched over from original engine.. Ever since motor was installed...when the low gas light comes on, or guage gets to about eighth of a tank- trans goes into Limp mode- put gas in it, get guage up and light off, Trans works fine. I swear this happens-lol... Check engine light constant, but scanner will not recognize computer, we can get into certain parts of the system, but not diagnose or read codes. the 3 x off/on key trick reveals no codes either. Then yesterday, on interstate about 65 mph, speedo quits working, trans quits pulling, until it slows down, then goes into limp mode. ABS and Brake light on, check engine goes OFF,all other guages fine. Tried gas trick,no luck. Suspected trans range sensor, or rear end sensor. Was in middle of nowhere, but found a pull a part, got as rear end sensor, but no trans sensor- pulled it out and cleaned it off, replaced rear sensor- drove about 4 miles in limp mode,no changes, then speedo kicked in, trans shifted normal, and check engine light came on. ABS and Brake still ON. Drove back home almost 70 miles like this, and about 10 miles from home, speedo quits, check engine goes OFF and trans into limp again. I've about had it with this Van, and cant get any info on scanner to help. Help? Ideas? Thanks in advance!!!!!!

My 02 2500 has had intermittent gremlins too. I got a FSM and cleaned/tightened all grounds and the BCM connector.
It was better for a while, but it's back.
Interior lights off and on (original issue), kicks out of OD, get the Check Gauges light, now it's running poorly.
I figure it's an electrical connection that has too much resistance or a flakey relay. I'm going to go through all the connectors I can find with some electrical cleaner spray. If I find anything that looks bad, I'll use the contact cleaner from GM. It's supposed to be the best out there but is not cheap. I found this, but I remember the stuff was a small bottle, not aerosol.
http://www.ebay.com/itm/Tronic-Clean-Contact-Cleaner-and-Degreaser-13oz-GM-4025-13-/230711154758

Turned out to be trans sensor AND rear end sensor gone bad at same time. Must have been causing problem for awhile, replaced both and ,all issues resolved, no check engine light, mpg improved about 3 mpg. Trans sensor was issue the whole time I guess.