If your hydrostatic transmission is locking up, it's possible that the brake system is malfunctioning when the pedal is pushed. My Sear DLT 3000 was completely immobile. It would not even roll when the transmission disengage lever was set. With the motor running, it would make a ratcheting sound and drag the motor when trying to move forward or back. I found that the brake lever on the tranny was sticking when the pedal was pressed. This lever is meant to immobilize the mower as a safety precaution.
Here's a bit more detail: When the brake pedal is pressed, which you must do to start the machine, a rod is activated that moves a small lever on the right side of the transmission. This lever was sticking on mine. Even when I reached under & manually released the lever, it stuck again as soon as I pressed the pedal to start. I just cleared a little debri from around the lever and it worked fine. I'll take a little more time now to clean and lubricate the lever.
BTW, I found this fix on another forum and wanted to spread it around in case it is of help.
