My little brothers 89 XJ.
Take a used empty oil/tranny and disconnect the hose and move it to a bottle, make sure it can move the nasty blow by down hill.
Have tried the remedy's (sea foam, Lucas, and a couple of others.) they do not work in the long run, they are just a band-aid.
Truly the only way is the rebuild it, like project88mj said to stop it.