Hi, I'm going to dig out the LATCH manual and type up the service bulletin/program numbers and tether anchor retrofit part details for you. The details also differ a tad depending on whether you're in the U.S. or in Canada, so I'll try to address both locations since I'm not sure where you're located, and it could benefit lurkers in either country who might have similar questions.
Ford's tether anchor retrofit program is Ford Program R7C for retrofittable pre-factory equipped 1989-2001 Ford models according to this .pdf document:
http://www.car-seat.org/attachment.php?attachmentid=19326&d=1357607813. The program is supposed to cover parts and installation labor in the U.S., and parts only in Canada according to info. recently posted on the CPSPList.
The LATCH manual lists 1996-1999 4-door Ford Explorers as retrofittable in all 3 second row seating positions, and the 1991-1999 2-door Explorer Sport models are retrofittable in the outboard second row positions. The 2-door version doesn't have a second row center seating position at all, only the 2 outboard positions. The Explorer's tether anchor retrofit part number is F17Z 78613D74-A. There are 3 drill dimples in the vehicle floor for installing the tether anchors in the 4-door model, and 2 drill dimples in the 2-door model. 2000 and newer Explorers have factory equipped tether anchors in all second row seating positions.
Chrysler/Jeep's tether anchor retrofit service bulletin is number 23-029-08, and Chrysler/Jeep dealerships can find additional information in the Warranty Service Center (WIC) article #1339. The program covers parts and installation labor in retrofittable models, and I'm not aware of any distinction between U.S. and Canadian locations in honoring the retrofit policy.
According to the LATCH manual, 1993-1998 Grand Cherokees are retrofittable in the second row center seating position using part number 04728952AB, but they're not retrofittable in the outboard positions. This sounds like what you had already heard about.
The 1999-2000 U.S. versions of the Grand Cherokee are not retrofittable in the center, but can be retrofitted in both outboard seating positions using part number 05016869AA for camel trim, or 05016870AA for taupe trim. The 1999-2000 Canadian version Grand Cherokees can also be retrofitted in both outboard seating positions using part number 05016871AA for camel trim, or part number 05016873AA for taupe trim.
If by any chance you're also considering the Jeep Cherokee model instead of or in addition to the Grand Cherokee, while I'm looking at the Jeep section in the LATCH manual, lol, here's the info. for the Cherokee as well.
1989-1999 Jeep Cherokees are retrofittable in the second row outboard positions using part number 050034422AB, and the tether anchor points are in the ceiling. Some 2000 model year Cherokees were factory equipped with tether anchors for the outboard seating positions. If it's a pre-factory equipped 2000 model year Cherokee, it can be retrofitted in the outboard locations using part number 05066385AA.
Happy car shopping and hope this helps you out.