You're gear has worn out.

Happened to me twice, easy to change if you have a spare, but the second time i said screw it, and got my cuz to TIG it for me and i grinded it smooth, and re-installed it. It has been good since. The track wears out the gear on the motor....

89 4Runner
3" BL, M/T locks, 33"bfg mt, bilstein, Kayline, tubebumper, toyotafiberglass panels
TBI: Elocker,3.4 w/ORS,b+b,S2Sstg2cams,arias pistons,P+P intake,TRDs/c,URDpullies+7th,downey headers,MAPECU2,WEGO WB, SupraMAF,walboro255,stg4clutch, EPaOo2 sim