My truck, after I first start it (whether it is warm or cold ..or even if I just shut it off to go to 7-11), seems to want to stay in 1st gear till I'm at something like 1800 rpm or maybe even 1900 or 2000.. UNLESS, I let of the throttle a little. Then it will go up to the next gear.

This behaviour seems quite odd, and has just started.

Truck has about 150,000 miles on it. It's be about 70K since a tranny service.

 

well there a few things .. is the trans stock. or does it have any valve upgrades

looks like it might need a new tranny.. but there are soem thing you can do.
there are lots of places to call and do.i seggest call dave at Georand Bros. http://www.gbtransmissions.com/
he helped me alot. my truck has been not wanting to shift into 2nd seince september of last year. but i finesse it into 2nd by letting of the go pedal. and dont abuse it to much. he sent me some parts after talking to me about everything going on.
worked great for a while , but i like to work my truck out in the desert!!and 150k on a trans is alot for most autos

 

Everything is stock. Tranny AND otherwise. No chips or anything.