The bouncing speedo is from the speedo cable binding up. I've had some luck fixing it on a few trucks by removing it at both ends and having a friend turn the trans end with it in a drill, while I spray some penetrating lube down the dash side of it.
Yes, there are differend gear sizes for the speedometer gears. I think it all depended on what size R&P the rig came with. I believe the speedo gears have the number of teath on the edge of them. See if your local Toyota dealer can hook you up with the right one.
If your 4runner came with a 4:30 R&P, you might want to change the speedo gear to an older truck who's lowest R&P ratio is a 4:10.
