This is a very simple but clever circuit and seems to be very accurate and stable.  It is actually not a real "RIT" but actually a variable BFO control with a +/- deviation of about 600 Hz.  The final result yields the same effect.

To affect this modification, you will need to sacrifice either your Tone control or the Squelch control (AM mode only), because the mod converts that knob/pot to controlling the frequency deviation.  I prefer to use the Squelch control because you can choose the 10K AM section and still have the control function as a squelch on FM because the other pot section is still intact.  This leaves the rather effective tone control still functional.  Here are the details (you'll need an R-2000 schematic):

If you chose to sacrifice the Tone control, C1 is disconnected from one end of the tone control potentiometer and a short piece of RG-174 coax or a twisted wire pair is connected to the end of the pot where C1 was disconnected.  The center wire of the coax then goes to this pot terminal, while the shield goes to the pot wiper which is at ground. If twisted wires are used make sure you use different colors for ground and “hot” differentiation.

Back on the main RF/IF board, capacitor C164 (27pf) is removed and replaced with a trimmer capacitor of about a 35 pf capacitance.  There are actually holes next to this area where it can be mounted and connected to the original C164 pads via jumpers.  I'm just guessing on this capacitance value because I was unable to read or measure the value in the sample R2K I evaluated. (This alone is a good mod because it allows proper alignment of both USB and LSB 1.5 kHz away from the IF center.)

Now connect (solder) the coax shield to a convenient ground point on the pc board and then connect the coax center wire to one side of a 15 pf capacitor.  Solder the other lead of the 15 pf cap to the "top" of T19, the BFO oscillator coil.  This node is also where one side of the trimmer cap that was installed is connected.

Now, if you choose instead to use the squelch control for the RIT as I did, unplug the 2-wire connector #38 and use the black and red wires from the pot.  You will have to short out the 2-pin connector on the main pc board because unplugging this cable will place the squelch circuit in a fixed “on” condition and shorting out the pins eliminates this.  I just cut off the connector and soldered the red and black wires together and plugged it back in.  Solder the black wire to a ground point and the red wire will go to one end of the 15pf capacitor.

Alignment:  Center the position of the tone or squelch control and then tune to WWV at 10.000 MHz (or another WWV freq) and push the dial lock on.  Set the mode to USB and adjust T19 for zero beat.  Next, switch to LSB and adjust the added trimmer cap for zero beat.  I found that I had to parallel the 22pf trimmer cap that I used with an additional 15pf fixed cap to get this part of the alignment to zero beat; don’t panic if your trimmer “rails out”, just shunt it with some additional capacitance.  You may want to repeat this step by going back to the USB mode and re-checking there, because there is a tiny bit of interaction between the coil (T19) and the added trimmer cap.

That's it.  When in either SSB or CW, the tone or squelch knob (whichever you used) will now function as a variable BFO control allowing approximately a +/- 600 Hz variation on the signal.  For a greater frequency spread, increase the 15 pf cap to 20 or 25 pf.  You will lose resolution doing this, however.

Standard disclaimer applies: If you feel your technical ability is below the level required by this mod, then don't do it.  Hire a technician to do it for you.  It would be a shame to ruin a perfectly good receiver.  Having a copy of the schematic is highly recommended before attempting this very effective mod.