Having defrost on turns the AC pump on, it's normal for the fans to come on with the AC pump is enabled.
You may have air in your coolant system!! This happened to me last year and had a small case of it in the winter, this is making me think that I have a leak in my head gasket.
To find out if you have air in your system, before you start it up in the morning, pop the hood and take the cap off the coolant tank and the radiator.
Before starting the truck, put your heat on HOT and fan on 4, this will prevent any issues when getting air out of the coolant.
Start the truck, the coolant in your tank may DROP to absolutely nothing, only if coolant starts to spill out of the radiator, cap the radiator and continue to let the engine idle. Watch the coolant tank, see if bubbles arise through that tank.... let it sit for a few minutes idling, if there are air bubbles coming out of the tank or radiator, wait till it stops coming out before putting the cap(s) back on. After all air escapes the system, you may notice that you are LOW on coolant, add coolant to the coolant tank until it stays steady between low and high. Cap everything, and you SHOULD be good! I've had to do this twice now within my past 7000 miles on the truck. I know when it has air in the system because the fan comes on more than it should and ONLY wen idling, the first time it happened, it was so bad, the engine temp on the dash would start to rise until I hit the throttle.
If this was the case, watch it every couple thousand miles, listen for your fan to kick on when idling again when AC and defrost are not enabled, if it does it again, it might be a bad head gasket... i may be wrong but I do need to figure my own out also.