I did a little experiment today, although not very scientific. I took my truck out for a 10 mile spin averaging about 40 miles an hour and then took temperature readings on each u-joint when I got back using a hand-held digital thermometer with a probe on it. The temperature outside was 78 degrees. Here were the results:
U-joint temperatures: (the rear driveshaft is one piece)
Back differential- 84 degrees F
Transmission yoke- 84 degrees F
Transfer case CV joint (two u-joints) - 84/84
Front differential- 87
Front axle driver side- 82
Front axle passenger- 82
I realize a longer ride at higher speeds will change these temperatures, but what I was really looking for is differential temperatures between the joints. In other words, if I had one going bad I would expect it to be running hotter then the rest. By this test I am concluding that at this point I don't have a problem. The front differential joint may bear some watching though since it was somewhat higher than the others. I'm going to track these for awhile as time will only tell if this is a reliable method to keep an eye on them. It sures beats dropping out the axles or driveshafts.