Ford Repair: ford transmission, ford f 250, ford transmission

Question
I have a 1994 ford f-250 4x4 w/7.5L 460.  Around 101,000 miles the trans began to slip.  No codes stored in computer.  It began to occur any time w/ w/o overdrive.  Had it rebuilt and the shop warranties it for 1 year.  Within one week it began to slip again.  The guy pulled it again replaced the valve bodies and put in a shift kit.  Began slipping again few weeks later occasionally but when loaded with 6000lbs of hay on a trailer it really acted up.  Began bogging down, no power, and top speed 60 mph(this load before was nothing for this truck) lost speed going up and down hills. Once i got the hay off no more problems. The shop owner is going to pull the trans. out again tomorrow.  The second time I took it back no codes found and nothing on the computer he plugged into the truck.  Any ideas?

Answer
I would make sure the speed sensor (in the rear differential) works-- if the speedometer is fine, so is the sensor.  Having checked that, I would make sure the trans shop does it right this time and also install a large transmission fluid cooler to help prolong the transmission.  In my experience, transmission rebuilders get it right about 1 in 3 times, and I have dealt with a lot of different rebuilders.  If you don't want to take chances the only way to go is a factory-rebuilt unit which is usually in the $2000 ballpark.  Most people would rather take their chances, and then all you can do is keep trying.  Hope this helps,
Jason