'99 silverado trans not shifting out of second gear - I own a 1999 Chevy Silerodo 2500 6.0L V8 Truck and I was out one night and parked it for ...

I own a 1999 Chevy Silerodo 2500 6.0L V8 Truck and I was out one night and parked it for the night and the next morning I went to go to town and it stay's in Second gear. I've checked the Transmission fluid on level ground when it was running and in park and the fluid is fine. My Four Wheel Drive unit works fine in Low range and in High range. My brother in law switch some relays under the hood to see if we had a bad relay and now I have a head light problem is there a relay that controls the shifting of the Transmission?

Your transmission has gone into fail safe mode. You will have to get a mechanic to install a scan tool and check for codes.

If its intermiten seem you might have a bad 2-3 Shift Solenoid. If you can catch the problem, the codes will be 081 and 082 which indicates shift solenoid circuit volage fault. Mine was so intermiten I never saw those codes.
This is what I done. Removed the tranmission pan and filter and identified both shift solenoids. Removed one solenoid at a time and using an True RMS meter measured the wires between the solenoid for 20-40 ohms minimum. I happened to measure my 2-3 Shift Solenoid at about 80 ohms. Just so you know, if both solenoids lose power you'll have 3rd gear only as a result...

Success on my part..... Good luck

fail safe mode.... deffinately... take it to a mechanic