gcp,
Just getting to this thread. Glad to hear that you got it figured out. I have the same/similiar problem with my IH584. All I have is 2nd, 3rd, and reverse in both the hi and lo range. Sometimes, it'll hold in 4th. I've always feared it was internal to the transmission. However, I do recall my uncle saying, before he passed, that the linkages just needed to be adjusted or repaired, (I wonder why he never got around to it). Now that I have the tractor though, I will do that once I return to the states. It has been run like that for so long however, I hope that some of the gears have not worn. I may get in touch with you later on for some advice. Thanks for posting!
Hawk