Mike,
Diesel fuel doesn't go bad nearly as fast as gasoline, but the fuel stabilizer is relatively cheap. I'd add some and top off the tank to prevent condensation, as previously suggested. My main worry about fuel would be algae if there is any possibility of water in the tank. There are additives to prevent this too.
I would not plug it in until I was ready to start it again. Keeping the engine warm will just waste electricity (about 750 watts) and keep the oil too warm. The warmer oil will run down the cylinder walls, and every other vertical surface, faster than cooler oil. You will want that oil to be there when you start the engine again.
I recommend removing the batteries and taking them inside (unless the truck is parked inside). Keep them charged, either with an automatic trickle charger that won't overcharge them, or with occasional use of a regular battery charger. A charge every two to four weeks should be enough. If this is impractical, find the IOD (Ignition Off Draw) fuse and pull it. Replace it before starting the truck. Or just disconnect both batteries' negative cables.
Now that I have a six-speed, the older, 1995 truck doesn't see much use. I haven't had any problems with it other than overlooking the batteries occasionally. I doubt that six weeks out of service will matter much to your truck.
Good luck with the recovery and with the truck.
Loren