On days colder than 45º F, it seems that my transmission will not allow the torque converter to lock up until the engine has reached normal operating temperature for a while. I doubt this has much to do with the engine operating temperature and more to do with the transmission operating temperature - I can leave the car for 2 hours or so and come back to cold engine (not completely, as it reaches operating temperature much quicker after starting), but the transmission will allow the torque converter to lock up much much sooner after the engine has started. I'm assuming this is due to the transmission cooling down much slower than the engine.

Is this normal, or do I have a problem on the horizon?

How many miles on your truck, how often do you service the tranny? At what RPM speed does your truck shift into 4th?

105,000 miles on the truck. Ive put 2000 on it since I bought it, so I cant say for sure how many times the transmission has been serviced before that. It gets into 4th pretty quick (looks like it shifts to fourth around 1800 RPM, but Id have to go drive it to check), but stays in 4th without the torque converter locked up until a few minutes after the temp gauge indicates the engine is at proper operating temperature. On warmer days, its fine.

Once it finally does allow the torque converter to lock up (everything is warm it seems), it performs perfectly fine.

90k on mine it acts exactly the same way, dealer said both engine oil and trans temp must come all the way up until the unit locks up. Im told it aids in two things, keeping the motor spun up helps get into closed loop faster, and less overall load on engine whilst cold when most wear happens.

That is what I was thinking, but wanted to make sure.

This is perfectly normal.
What ryanD2 said is right, needs to get to temp, helps in longevity and helps to reduce emissions.
A hot engine is much more efficent than a cold one, by up to 50%.

Lambchop,
What year/model are you driving??
The D2's are electronicly controlled by their own ecu and normally, with stock size tires, shifts into 4th and lock up at about 48 MPH when warm. If it is a D1 and the shift cable is not set properly, no telling where it is shifting.
Which ever you have, go change the filter and fluid, adding a bottle of LUcas to clena up the valves. Whaever you do, do not have the tranny flush at your mileage, it could kill it.

I have an '02 Disco II. The transmission shifts perfectly fine, it's just the torque converter that doesn't lock up until the tranny is warm. I figured it was normal, I just wanted to check and make sure.