I can't edit my original post, but wanted to update to say that I stopped in to talk to the parts dept. at our local Jeep dealer this morning. The guy knew what I was talking about but wasn't sure if it was available for a '98. He remembered it having come up before for another customer and said he thought he remembered some sort of universal tether anchor but that he really wasn't sure. Anyway, he said he'd research it and get back to me in the next couple of days.
After reading more on this forum, it looks like tether anchors aren't available for pre-2000 models. I'll be curious to see what the parts guy comes back with and what anyone has to say here.
ETA: Now that I can edit . . . the dealer called back a few days ago to report that they were not able to come up with anything, so I guess we are out of luck. I'm leaning towards getting a Radian or two since they are supposed to be good w/o a top tether. We can get rock solid Britax MA and RA installs with the seatbelt, but I really would prefer to use those seats with the tether. And the RA expires soon, so a-shopping-we-will-go.