there are two TCC related circuits on the transaxle connector, one for TCC PWM, the other is just TCC. you'll want to "intercept" TCC. it should be a tan wire, black stripe, in position "D". if that circuit is open, the TCC will never apply. if closed, will only apply the TCC when the PCM wants it applied.
manually shifting between 3 and 4(or D/3 or D/OD depending on how your shifter is labeled) is no problem at pretty much any speed. manually dropping into 2nd is.... rough, to say the least, and i really wouldn't recommend it unless necessary. it may or may not be damaging to the transaxle, but it is definitely damaging to trans/engine mounts.
for maximum engine braking, i have my tune setup so that when i lift my foot off of the throttle, the TCC won't automatically unlock until it gets down to the MPH threshold to unlock in a 0% TPS situation, which is around 36MPH in 4th and 26 MPH in 3rd. TCC staying locked alone helps a little, but i also have DFCO allowed to happen in the same instance, combined they produce quite an effect.
also, interesting tidbit: 1st gear is the only gear the PCM can actually force in/out... due to the way the manual shaft is setup on the 4T60E, if the PCM is commanding 4th and the shifter is in 1st or 2nd, the highest gear it will actually achieve is 2nd. shifter in 3rd, highest achievable gear is 3rd.
anyways, that last part is just trivia really, but useful in certain situations, like if the trans loses all power to the shift solenoids for some reason, moving the shifter between 2 and 3 will get you 2 forward gears instead of just 3(3rd because when both solenoids are de-energized, 3rd gear is being selected).
1995 Monte Carlo LS
3100, 4T60E, OBD1 Conversion...for now, future plans include a 3900, T04E-46 (Knock-Off) turbo, and a F40.