You should do this with the table saw. This is a very typical operation in the normal milling process.
Normally you would set the table saw's rip fence to the final width you need, but in this case, since you want to match an existing dimension you should ignore the numbers and use the physical dimension you want to set the fence. Put the board just ahead of the blade, so that there is just a bit of overlap with the blade with the narrower side of the board forwards. Move the fence until the edge is just brushing against one of the teeth (preferably a tooth with the tall side closer to the fence.) Lock the fence down and run the board through.
(Safety Note, any time you're working close to the blade the saw should be unplugged.)
I think your issue came about because you jointed both edges of your board. This should never normally be done, since, as you've seen, this will not ensure that the two edges will be parallel. Here is the typical milling process:
- Face-joint one face so that it is flat on the jointer
- Make the other face parallel to the first face on the planer
- Alternate planing both faces until the board is the desired
- Edge-joint one edge so that it is perpendicular to the faces with
the fence on the jointer
- Rip the board to final width on the table saw, which will ensure that the
second edge is square to the faces (since the blade is not tilted)
and parallel to the first edge (since the rip fence is a fixed
distance from the blade).
Furthermore, using a router table to do this is not the best idea because it would require you to "pinch" the workpiece between the bit and the fence. This is a bad situation because it sets you up for stalling or kickback. (Note that offset fences and a straight bit aligned with the outfeed side won't work here, because it will cause the same issue that the jointer did.)