I just had the 350 re-built after 300k last week in my 89 suburban , but I started noticing that after the motor got done the truck stays in 1st gear for a while and under acceleration as it shifts theres a nice clunk that comes from the trans, but if you let off the gas right before it shifts theres no hard shift. I tow my race car about 120 miles round trip and make the trip once a week. theres quite a few steep hills, I was debating having it re-built anyways but is there really a need? It is a good wrench transmission installed in 2001 but I have lost track of the miles. I got quoted at 1600 to have it re-built with all the heavy duty gears, bands, etc.. Since the motor has 500 miles on it I figure why not get the trans done as well.