Help! Car stalls when put in gear.

I thought about that after I talked to U-haul. I'd like to avoid that if possible since I've got another car and my 3 bikes on that policy, and this would be the third time they've towed the Mustang. And, of course, I'd still need it towed back, unless I had the shop fix it. They won't even give me a range for an estimate, claiming there are multiple things that could cause this. That's why I'd like to consult a trans expert to find out what different problems could cause a trans to lock up like this. Taking it to the trans shop would cost me a minimum of $213, which wouldn't cover actually fixing it. I have a strong feeling that after their diagnosis, they will tell me I need a new torque converter. Although I hate just throwing parts at the car, hoping something will stick, I can get a new torque converter for $190, and a new transmission jack at Harbor Freight runs $125, which I might be able to get on sale. That makes it tempting to just put in the new converter and hope it works. In addition, the muffler is shot and needs replacement anyway, so I can do that at the same time. Since I didn't know what a crappy trans the A4LD was before this all started, if I could find a donor car, I'd convert this to a 5 speed.

Update. In my search for info, I found some very good explanations of how the torque converter lock up works, along with a Ford A4LD Technical Service Bulletin Update Handbook by the Automatic Transmission Service Group (ATSG). The documents explained that the converter clutch would always be locked up by its clutch spring, just like the pressure plate on a manual. The TCC and 3-4 shift solenoids apply fluid pressure to push the converter clutch back. When the EEC interrupts the circuit to the solenoids, the springs on the valves underneath them push those valves to exhaust the fluid away from the converter clutch, allowing its spring to engage lock up. If those springs, the converter clutch shuttle spring, converter clutch TV modulator spring, and converter clutch shift valve spring, break or weaken, the car will shudder as it slows, or even stall, because the converter lock up will not disengage, much like a manual car if you do not disengage the clutch as you come to a stop. They advise replacing those springs and in the case of the converter clutch shift valve spring, they said it was reduced from 5 lbs to 2 lbs when Ford went from one solenoid to two solenoids. They suggest putting in a stronger spring between 4.5 and 5.5 lbs, or i.e., just put in the original single solenoid 5 lb spring. Unfortunately, those springs are discontinued from Ford and difficult to find. However, TransGo makes a reasonably priced shift kit that includes those springs, along with several others, that are supposed to improve shift feel. Their converter clutch shift valve spring is 3.5 lbs, not as much as advised but much stronger than stock. The kit, along with necessary gaskets, is available at a shop not too far from me. Therefore, Sunday night I will jack up the car and start draining the fluid and pick up the shift kit Monday. That reminds me, I am sick of the mess when draining trans fluid so I'm going to install a drain plug kit while I have the pan off. I'll update once I've installed it and buttoned everything back up.