In my experience, racks are interchangeable as complete units, regardless of who made them. I put a TRW rack from a 90 on my 88 that originally had a ZF. Apart from a minor difficulty getting all the air out of the system (which solved itself after a few days when I turned the wheel abruptly while driving at moderate speed) it worked perfectly. However, be aware that components, such as tie rod ends, boots etc., are NOT interchangeable between brands.

Stiff steering can be caused by a tight universal joint in the steering column. Have you checked this?