I don't know how much they have changed between the 97 and 98 years. I have a 96 Z71 and I am aware of the problem of the 4L60-E transmission shortcomings. You don't say how your Tahoe is driven, or if it is used for towing very often. Check out this other post I responded to:
http://www.automotiveforums.com/vbul...d.php?t=273822 The clunk could be anything really from the transmission starting to go, to having a u-joint just start to fail. If the u-joints have never been serviced and you have over 135,000miles, it's not like it would be throwing money away. I would start there first. As for the tranny issue, the only thing I would recommend doing, would be to get a fluid and filter change done on it. There might even be a Technical Service Bullitin issued for the Tahoe also concerning the u-bolts that secure the rear axle to the leaf springs. Hope this gives you some ideas -Chris