Trans. not shifting into OD. - Anyone have a clue as to why my '96 Chev. K1500 350 won't shift into OD until the vehicle has been driven a while ...

Anyone have a clue as to why my '96 Chev. K1500 350 won't shift into OD until the vehicle has been driven a while and is up to normal operating temperature? I've asked online mechanics about this, but never really got a solution.

You didn't say how many miles on this transmission. It might be a good time to drop the transmission pan and change the filter and refill with fresh fluid to replentish what you lose while draining the pan. While in there, see if you find excessive clutch material or shavings in there. If everything looks good inside and you still have the problem, it could be a bad seal within the overdrive hydraulic circuit which "seals up" with temperature. To fix that, it will require professional help.

At the time the problem started, I had maybe 65 or 70K on the truck. Changed the oil & everything (the dealer did it) at 60K.