I have a c6 transmission with transbrake valvebody,when you shift from 1st to second gear the rear tires lock up in second gear ,all other gears work fine and transbrake works fine also.

If it still doesn't want to move, then the transmission has to come apart and I would suspect you have a broken shifter fork.

we can not drive your car or see it fail or hear the tires skid to a stop
ive no idea what is lockup. means. (not really) unless one tells what happens step by step and all evidence.
good and bad both.
car stops moving,after moving in 1st gear or not at all
or skids shifting to second in drive.2nd.
rear tires skid?

if the rear axle on the car locks up, in motions say so.

Don, Possible internal damage to transmission, take into a good shop for diagnoses and estimate on repair.
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I don't know if this will help,but i set my tranny up to shift hard like you are describing. Just by putting a clip on the kick down cable. The book talks about this, its cool if you like that sort of thing?

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/jeff_24365d731679262d

sensor plug with white band goes to the outside sensor, they will plug in wrong and cause the same sympyom

You have a shift solenoid sticking its located in the valvebody.Pull the transmission pan off and you will the solenoids in the back of the valvebody they will have wires hooked to them.Replace the bad solenoid and the filter.

disconnect rear drive shaft to see if tires move if not the rear end pinion gears might have a bad bearing causing the lock up.the fastest way to repair it is to replace gear box with good used one from a salvage yard.the reason I'm telling you this is because although your gear box might be repairable, it would have to be done by a professional and cost would be high.

Tighten the band adjustment to 18 inch pounds it is a stud with a locking nut on the side of the transmission it is the one located toward the rear of the transmission not the one towards the front that is the 1st/reverse band adjust replace the modulator if problem persists sometimes they stick

What you need to check is the transmission release bearing because if there is a clearance on the release bearing you will encounter this problem and also check the transmission oil.

I’m happy to assist further over the phone at https://www.6ya.com/expert/ramy_d9e7ebea3891f482

transmission valvebody is stuck