Towing a 12K lb fifth wheel trailer thru the heat in Kansas yesterday - both engine and trans temps remained in normal range - pulled off the interstate, felt a significant 'clunk' when slowing to a stop. Check engine light came on and could hardly accelerate, then slowly returned to normal and I continued another 200 miles with no problems. This morning, check engine light was out, drove about 100 miles, pulled off the interstate - same clunk, loss of power and engine light comes on again. However, now it is stuck in second gear. I crawled about 20 miles to the nearest GM dealer and am waiting 'til they open in the morning.

Any ideas on what may have happened ?

Totally stock 2005 GMC Sierra pickup 2500HD with 6.6L Duramax, Alison tranny, 15K miles - trans filter changed at 8K miles.

If you are stuck in 2nd that usually means something happened in 3rd. Need codes. Have you changed your spin on filter yet?
mike

Changed the filter about 7k miles ago. Will get the codes from the GMC dealer tomorrow am.