After checking the connections and they are ok, I'd pull the alt and disassemble the back plate. If the slip rings are chewed up just replace it. Reman or junkyard is good. I prefer to go to the yard and pull an alt out and check it's condition. The factory usually does a better job building things than a reman outfit. And most vehicles at a junkyard are wrecked (possible low miles) or just old, but probably not offroaded and abused.

91 YJ
2 Seater Sand Rail
Handfull of other fun toys...
And a brand new, spit shined, B.S.
Oh, and a job to boot.