Here's an update. The other day the response time for the gauge was starting to lag again so I put another two bottles of fuel injector cleaner into half a tank and drove it till the low fuel light just started to light up. I filled it and this time the gauge didn't move up all the way, it only moved slightly to the point where the light was off.
I just removed the sending unit last night and yes its very easy to remove. Just unplug and remove that big nut. There is a small hole in the aluminum tube that will allow the fuel to drain out as you remove it, takes about a minute or two, depending on how much fuel is in the tank. Access from the trunk is very good.
Once I got it out, I didn't see any nuts or screws or anything that the others have posted about diesel sending units so I assume they must be different. What I did was straighten out the three tabs/crimps at the base of the sending unit and ease out the black plastic cover to access the float and contacts. That was a far as I could dismantle. The top of the sending unit was crimped in pretty well and it didn't look like it would come apart easy. I took a bottle of Valvoline carb cleaner and sprayed the float, the rod that the float rides on and everywhere I saw the "yellowish" residue. I musta used about half the bottle and got it to the point where when I would slightly tip the unit upside down the float would start moving, when I first started the float wouldn't even move when it was upside down unless I jerked it slightly. I put it back together and put two new O-rings on the nut and tried it out. It responded right away and this morning when I filled up it worked right away. If anyone knows how to remove the top of the sending unit subassembly from the whole unit, I would be interested to find out, but as it stands now, its working and hopefully it will for awhile. Total time about 30 minutes. I went to http://www.carpartstogo.com/
for the O-rings, about $5-6 for the two and they do sell the sending unit for about $70 for the 190. Ken