Nasty Slamming Transmission Noise
 

I have a 2000 4Runner SR5. For the last month or so it has been slamming into gear when I go from reverse to drive. I will press the accelerator and it will hesitate then slam into gear. Sometimes it will slam three or four times before moving forward. At other times, it will slowly drive forward and not slam. When I am sitting at lights and let off the brake, it seems to click into gear.
Temperature and position have little to no impact.

Anyone have any clues as to what this could be? Multiple mechanics and dealer mechanics have found nothing by visual inspection.

Thanks,
Vic