This is the same ClockworkMod Recovery 126.96.36.199 as in this post, just with the kernel patched to run on phones upgraded to Android 2.3.4.
Windows users should download and install One Touch Upgrade Q (it includes the needed Windows drivers). Linux and Mac OS X users should download and install the Android SDK starter package, then use the Android SDK and AVD Manager to install at least the Android SDK Platform-tools component. Linux users should also configure the system as described in the Google Android development documentation.
Download the attached file: recovery-clockwork-188.8.131.52-ot990-kernel-v53B-0-anyboot.zip, unpack it.
Switch the phone into fastboot mode:
disconnect the USB cable, turn off the phone;
press and hold the Volume Down button;
while holding the Volume Down button, press the Power button;
when the phone vibrates, release the Power button, but keep holding the Volume Down button;
keeping the Volume Down button pressed, connect the phone to your computer with the USB cable;
wait 3 seconds after connecting the USB cable, then release the Volume Down button.
If using Windows, just run the install.bat file included in the archive. For other systems you need to run the commands manually:
fastboot erase recovery
fastboot flash recovery recovery.img
Entering ClockworkMod Recovery
Disconnect the USB cable, turn off the phone.
Press and hold the Volume Up button.
While holding the Volume Up button, press the Power button.
When the phone vibrates, keep holding the Volume Up button, release the Power button, and immediately press and hold the Home button.
Continue holding both the Volume Up and the Home buttons until the Android logo disappears, and you see a black screen with the keypad light turned on, then release both buttons.
Wait while CWM recovery boot completes; you should see the menu as shown here.
WARNING: If you forget to press the Home button, or release it too early, a hardreset will be performed instead of entering the recovery — it will wipe all user data from the phone memory (but files on the microSD card will not be affected).
When working with the CWM recovery, use the Volume Up and Volume Down buttons to move between menu items, and the Home button to activate the selected item. Use the "reboot system now" item to exit the recovery.
Rooting the phone with ClockworkMod Recovery
Download Superuser-3.0.7-efghi-signed.zip (or maybe a newer version from the Superuser home page).
Place the downloaded file on the microSD card.
Enter CWM recovery on the phone as described above.
Select "install zip from sdcard" in the menu, then select the Superuser-3.0.7-efghi-signed.zip file.
Select "reboot system now" to exit from recovery.