89 Trooper II transmission problem

My 89 Trooper ,4 cylinder engine, blew head gasket few months ago. I replaced the head gasket and the engine runs beautifully but the transmission stopped down shifting to first gear from the park position. I was able to down shift using shift level and then when once the vehicle was in motion I could put into drive position and it would drive normally. As the weather got cold the tranmission stop down shifting all together from stop. I took it AAMCO transmission and they had it for few days, then told me could not find the problem. I took the car to a very reputable shop in my area. I had a call the next day and they explained to me the problem is caused by a faulty head (the sending unit in the speedometer) . They had to order one from Japan since no dealer in US had one. This was a month ago and now once a week I call the shop to find out when I get car back and they say they are still working on it and can not find the problem.
any ideas Thank you all