'99 4Runner,4WD, Automatic 6 Cyl, 220K Miles,Rebuilt Trans 2 yrs ago. Acting same as it did when I had tranny rebuilt -Usually at start will reverse but won’t engage in any fwd gear. Then slips (more like ‘slams!’ into gear like it has vacuum leak. very infrequent (1X/mo) but this week it did it while at a stop sign after I’d been driving a while. I know I have a Rear-Diff leak. Is this a tranny issue, rear-diff or something else?