Chances are your REAR tranny mount (bolts to a brace just above the rack/pinion) is no good. You is a pain to get to and change if the motor is in the car. I had the same problem, turned out to be that mount. Check all your other mounts as well. If you have a 92+ car, there are 5 mounts total. Upper drivers side motor mount, lower drivers side torque mount, upper tranny mount, lower tranny torque mount, and rear tranny mount. Check them all. What seems to be happeneing is that the motor is pulling AWAY from the axle and causing it to pop out.
The other thing you want to check is the C-Clip on the axle. Make sure it's opened enough. When installing the axle, you should heard a pretty obvious CLICK. The ONLY other thing it could be is that the groove that the c-clip sits in could be worn and not seating the c-clip correctly. Anywho, good luck!
