Found this while surfing and thought you guys should read it. I haven't torn down many 4L60E trannys, except those that were overheated and toasted by morons who overload/tow in OD and don't even have a trans cooler. I think I will do this one myself next week when I change the fluid/filter. List price for the kit (includes boost valve and sleeve) from GM is $10.96.
4L60E Maintenance
Also on the same page is a good explanation of why you shouldn't tow in OD. I will post a link to that in towing for those looking as it seems to be a repeated question.
Later