2000 Silverado 1500 I purchased @ 77 k miles.

Seal went out at 103 K miles. Dealer replaced on extended outside warranty.

Seal went out at 134 k miles. Had a specialty rear end service outfit replace.

Seal went out at 139 k miles. Same service replaced it again along with right rear axel, both axel seals and new crush washer.

Question: What is considered normal temperature for a rear end to run?

This is not a common problem with these trucks.
Possible the the axle vent is plugged up or????

I never heard of a "normal" temperature but on some models they used synthetic gear oil due to the heat so it must get right up there. The type of service/load would have an effect I would suppose.

I would find a different dealer and ask him what would be normal for seal leakge.

You may want to check and see if you have a driveshaft that is made with composites. I have read where those shafts start to come apart and the shake is slight and you might not feel it, but could be enough to damage a pinion seal. I'd also have the pinion bearings looked at next time the seal goes out.