Little coolant drip
 

99LS 4dr

Flushed out the cooling system last week. Today I noticed about a 2ft discoloration under it in the driveway. Looks like a drip of about 1 drop or less per minute, dead center under the engine. I can't see where it's coming from, but everything around the flush tee and above the water pump (heater hoses, top part of gasket, etc.) is dry.

191k miles, don't know if the pump has been replaced before. Should I even bother just changing the gasket, or put a new pump in while I've got it off?

If the water pump leaks it most always comes out the hole near the bottom center of the pump. The hole may be hard or impossible to see. Some vehicles it can be seen from the bottom. If the leak has started slowly over time as they ususlly do you should be able to see a stain coming from the pump and going down the lower radiator hose.
The gasket could leak but not likely.

If it is leaking from the dead center on the front of the engine then it is likely coming from the weep hole in the water pump meaning a new pump is in your future.
If it is about midway or further back then there are a couple other possibilities.

The first would be a slow leak running down from the heater core hose fitting exiting the block on the passenger side. The fitting here tends to corrode over time causing slow leaks. The coolant will follow the curve of the engine and drop almost dead center of the truck.

The second would be the very common leaking of the intake manifold gasket. The OEM gasket pretty much disintegrates to nothing after a while and the coolant flush may have dislodged gunk that was keeping a leak at bay until it was removed by the flush.

Check for coolant in your oil and or a slight odor of burning coolant in your exhaust. Though if a leak in your intake manifold gasket is still small enough these signs may not be detectable just yet.

If you do swap out the pump you will find that it's a pretty basic job for the home mechanic.
I would go with a new (not rebuilt) water pump. While you are at it... check those hoses and replace the thermostat with a quality unit.
I would also look into changing the serpentine belt and tensioner since you have it off anyways... cheap insurance.

After pricing pumps, whether I do it myself or not, that critter's getting changed. $60 for a Bosch or Delco is too cheap to waste time having to do it again later.

The belt and most of the hoses have been replaced in the last year. No idea why they didn't change the pump, though.

That doesn't surprise me that they didn't change the "non-leaking at the time" pump... I wouldn't have either. Good luck!

I guess I just have this theory that, if it's ten years and nearly 200k miles old, cheap, harder to remove than the oil filler cap, and I'm going to have easy access to it during another repair, I'm going to change it. If they had the serpentine belt and heater hoses off, (and pretty much had to have drained all the coolant, since the top and bottom rad hoses look pretty new too) they were more than halfway to changing the pump.

Definitely get a new pump and thermostat! Its not that expensive, and it will save you hassle in the long run.

On closer inspection, it looks like there's coolant residue on the front bottom of the pump, so it probably is leaking there. OTOH, I put some stop-leak in it the other day, washed the engine off well, and didn't see any more drips last night. It's still on the to-do list, but for now I can take it off the must-do-right-away list and hold off until finances allow for a few other things while I'm getting dirty anyway.

right on man. But just watch it. That is a temporary fix.