ok- I am still using the lift, so I have to put it back together after each diagnostic session---I've been collecting information because I want to do as much as I can in each session. I think I can spend some quality time with the unit in the next few days, and I will test the functionality of the various components as per the manual and instructions I have received. I will update this post after I have done some significant testing. Thanks to everyone for your suggestions!

just to clarify 'control valve' i mean the transmission control valve not the hydraulic control valve.
and yes if something went wrong in that 'transmission' control valve it could effect just about anything controlling the transmission function.
But my origional comment was intended to guide you towards checking the electrical side of the directional circuit, starting at the control solenoids on the transmission control valve and work your way backwards to the directional switch.

It's not a monotrol. Is there anything in the control valve assembly affects both clutch packs and the inching pedal? The bias is definitely towards the forward gear.

even though it could be a possible clutchpack issue i would start by checking out the control valve.
- signals to the solenoids (make sure its not trying to engage both at same time while in reverse)
- if it is monotrol then check the pedal assy real good

then take pressure readings

According to the tag, the delivery date was in 1997, and the S/N is D187V04384T. I added my email address to my profile.

Do you have the Serial # of the truck in question and also do you have a email to post in your profile in case anyone wanted to contact you?

I haven't checked the pressures. I probably need to get a service manual before I go much further, because I don't know how to do that. I am a decent mechanic, and I've changed mast seals and rebuilt the top end on another lift, but I may be in over my head on the transmission. I can try draining the oil though. Also, one other symptom: the inching pedal works, but it seems like it isn't engaging fully. If I rev the unit with the pedal depressed it will overcome the brake and move forward. Thanks for your help!

If the forward clutch pack has warped or broken discs and is dragging bad, thats the symptoms you get. One quick test you can do is drain the oil out and start it. If it still wants to move forward, while in neutral, it definitely has a damaged clutch pack.

Sounds like it may have stacked the packs in the transmission. Have you done pressure checks to see if the pressures are correct?