Have you tried to up in 4-hi? If you were lugging it in 4-lo you could've overheated the transmission. 5k lbs is not too heavy to try to make it 4-hi. I would try that. I friend of mine who was racing at some dunes in 4-lo overheated the transfer, and it wouldn't go back into 4-lo. Maybe the transfer overheated. Next time go into park and then back into drive. Sorry I can't help.