Glow Plugs

Change them all and be done with it....some risk in leaving a bad glow plug.

You are lucky it's just glow plugs. You don't even want to know what replacing injectors costs...and you have to do them all as a set.

I have the same engine as you. I had one burn out around the first of the year. I just got around to replacing it last month. I just burn out another one the other day. If I have time I will replace it this weekend. I would replace it when it gets cold though because "I" don't like fuel knock or rattle.

There is no danger to let it go as you said other than hard starting when very cold.

As DiskDoctr pointed out other things can happen but they are unlikely. But I would get them replaced because it will make for harder starting in cold weather which puts a further strain on your starter and batteries.

They aren't difficult to do on the LBZ but if you aren't the do it yourself type have the dealer do them. If you don't get them replaced make sure to plug in the block heater for a few hours when the temperature drops to 32 or below to make starting faster; it will still start with bad glow plugs and no heater in even colder temperature but it will be an ugly start.

Note if you use the block heater it should be on for a few hours, don't just plug it in for 1 or 2 hours especially if the temperature is at or just above freezing. The LBZ compares temperature readings from several sensors and plugging in the block heater for a short time when the temperature is NOT well below 32 may trigger a CEL as I found out right after I bought mine new in December 2005. The sensors do a cross check and use of the block heater for a short time will fool the system into thinking one of the sensors isn't reading correctly. The CEL can be reset by a reader or it will reset itself after a few normal starts; the only operational change I found when it was on for this issue was the transmission wouldn't shift into 6th (second overdrive ratio).

Damaged glow plugs have been known to swell and require removal of the heads to replace them ($$$) or even break off into the cylinder (boom, or $$$)

It is dependent on the vehicle, of course, but Ford OEM glow plugs are under $10 each.

Labor would be there if you don't do it yourself, but I wouldn't let it go.

Now if was your glow plug control module or something electrical and you decided not to bother fixing it, that would be different than a possible physical failure that it sounds like you may have.

It's up to you, I'm pointing out the risks that could be associated with ignoring it. Risks are not guarantees, either ;)