85 -mid 86 used the TRW cam gear rack, 2.75 turns lock to lock.
mid 86 - up used the ZF rack which is 3.2 turns lock to lock.
The TRW and ZF refer to the different manufacturers of the steering racks. So in theory you can tell which one you have by sitting in your car and counting how many times your steering wheel goes around from one side to the other.
The racks can easily be identified by looking at the rack tube. The Early TRWs are smooth across the length of the cylinder, whereas the ZF racks have a waffling pattern stamped into the casting of the tube on the passenger half of the rack tube.
FWIW, ZF racks can be interchanged between the Scorpio and XR, by changing the inner tie rods. The racks are the same, Scorpio tie rods are about 3/4" longer. TRWs can be replaced with ZFs.
I ordered the parts from Autozone, which sells ATSCO. They had the cheapest pricing (not generally the best criterion, I know...). For the rack, I called ATSCO and they said they can supply either the ZF rack or the TRW rack. ATSCO calls the ZF rack the "Type 2" rack. The TRW is called "Type 1".
For example, the way to get the ZF rack from Autozone is to have the sales person phone ATSCO at the time you place the order. I had no problem getting them to do this. The Autozone sales person should tell ATSCO they want the Type 2 ZF rack. ATSCO will then link the order and the rack type to the PO# which Autozone also supplies.
ZF Rack: ATSCO p/n 6441 Type 2 $139.99 + $90 core
TRW Rack: ATSCO p/n 6441 Type 1 $139.99 + $90 core
About a year ago I developed a leak in my steering rack (1985 XR4Ti), which I noticed because the steering boots were accumulating steering fluid. This was a rebuilt rack with maybe 30K miles on it, installed by the previous owner. My initial inclination was to replace the rack. I found that a replacement rack was expensive, and an internal "repair kit", even if available, was also expensive. I removed the rack, and disassembled it. I guessed that the leak was from one of the two end seals (one seal at each end of the rack housing). I was unable to find these seals at the usual sources (Ford/Mercury dealers, Rapido, car parts suppliers). I ended up buying the seals at an industrial supply house, ordering by size. The seals cost about $6 each. I installed them in the rack, reassembled it, and reinstalled the rack in the car. I was lucky. One of those seals was apparently the source of the leak, since I have had no steering leaks since then (about 20K miles). There are of course additional seals in the pinion part of the rack assembly, but fortunately for me these were not leaking.
The size of the rack end seals (2 required) for my 1985 XR4Ti are: 24 33 7 referring to the inner diameter, outer diameter, and thickness, respectively, in millimeters. Most industrial/pneumatic supply houses can provide such seals.
An additional detail, which may not be required in all cases: While I had the rack apart, I made new "support washers". Their purpose seems to be to radially locate the ends of the rack as it slides back and forth through the end seals, so that the rack end stays well located in the center of the rack bore, preventing the rack from deforming the lip of the seal as the rack moves. I made 2 new washers by machining a rod of a material called "Delrin" (trade name for a kind of durable plastic; plastic supply houses can supply this, it's very common). I made the new washers a bit thicker than the original ones, to hopefully allow them to better support the rack ends and wear longer. If you don't have a lathe, I think a machinist could make a bunch of these washers to your measure in an hour or less. You just chuck the Delrin rod in the lathe, bore a hole in the center, and turn the outer diameter to size. Then slice.
I think that the cause of the leaking was that the old support washers had worn and allowed too much radial movement of the rack against the seals, thus deforming the lip of the seal(s) a bit, and allowing fluid to seep past. Perhaps this is a general failure mode often causing leaks in these racks? Just my theory.
I feel fortunate to have been able to solve what could have been a multi-hundred dollar problem for about $20 plus maybe 6 hours time.