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Did you reinstall all the metal shims (flat washer thingies) in their correct order? Also, make sure the headlight motor case is sealed correctly. Sometimes it can appear to be sealed flush, when it fact it isn't.
'84-'87s don't have the a shim/washer set up like the '88s and up; there is only one washer on each each side of the black gear. However, they do have a metal 'clutch' assembly that sits on top of the white gear.
There are two two pieces to this; a flat piece that sits direclty on the white gear and another piece, with to tabs that stick down. If the tabs of this piece are not properly aligned between the two black 'stops' on each side, this could be causing the problem. Have you checked to see if they are? What do the black 'stops' look like when your motor goes out? Are they chewed up pretty badly?