My 98 XJ has abrupt shifts between first and second.
After not having any luck with fitting a new Transgo valve, I'm thinking about swapping out the valve body with a used one. It seems pretty straight forward unless I mistakenly open the valve body up. Does anyone have any words of wisdom / advise on the subject?

 

If you put the transgo in you already removed the valve body, so swapping is no different. But you don't know for sure how good the replacement is.

 

I guess the bore for the valve could be too worn for the Transgo to work?
VAlve body replacement is probably the only remaining fix that I can do myself.
What else would cause the rough shift between first and second?

 

I would agree switching the valve body is the best and fastest option. I installed a Transgo valve kit also and was stumped that it was having the same slipping problem on my Mercedes. I got a junkyard valve body for cheap, installed it and the same problem occurred. Now I know I have a slipping clutch pack!

 

What is the fix for a slipping clutch pack?

 

Well I installed the valve body and it made no difference.
Next step, hunt for a used transmission.

 

Before you try that, make sure you use the correct ATF and that it is filled to the max and drive around again. Hard shift is related to valve body, not the tranny. If you still have problems, disassemble the valve body and completely clean out carefully. Also check every solenoids for proper operation by hooking up a 12V battery to it and blowing air through it with a rubber hose (I use a Prestone coolant tester, the one with a rubber bulb, instead of my mouth). If possible, if you have the info, check the ohms resistance.

Slipping clutch pack situation - assuming correct fluid, level and proper valve operation, then it is either worn clutch friction material or clutch piston leakage.