1998 Jeep transmission

Hi I have a 1998 Classic Jeep with 165,000 miles.
I can't seem to get a mechanic to figure out what
the problem is. Bought in Feb.2006 and drove fine
until early summer then it would not go into OD.
My mechanic replace a switch and it was good again
until 1 1/2 months ago. It would not "pull" the engine runs great, but it just will not go when put in gear. Had it towed to a shop the mechanic drained the trans fluid and fixed a leak. It drove great for about a week. Then it happened again. He didn't know what was wrong and I've had it a two more places and they don't know what is wrong. They have checked the codes, but said the codes didn't tell them anything. It would get so far in the test and then not go any futher. If I start out in 1st and manually shift up it works, but if I start out in OD it
doesn't want to go. Can you help head me in the right direction with this problem? After three mechanics and still no fix.I'm not sure what to do.