There is an HD version of the 4T60E... I think the major differences are a beefier differential and housing, and maybe some different clutches. My memory is escaping me right now. But it'* called the 4T60E-HD and was mated to the Series 2 L67 in 1996 and the Series 1 L67 from 93-95 or something like that. In 97 the L36 was mated to a regular 4T60E, but the L67 got the 4T65E-HD. From '98 through 2005 everything got the 4T65E (L36) or 4T65E-HD (L67). So basically if you're looking for a 4T60E-HD you'll want one out of a '96 or prior SSEi or supercharged SLE/SSE.
Someone please correct everything wrong with what I just typed.