Inside the valve body, under the pan, you will see all your solenoids etc, (except duty/line pressure solenoid etc in rear housing), you will also see there is a small (yellow?), wire that earths all these solenoids, generally if you have an earth issue, it will affect ALL the solenoids so you can rule that one out!. The resistor you guys are talking about as I understand it, drops the voltage to the solenoids/auto to 10v for better solenoid longevity. But I don't think it will affect solenoid operation, unless it fails completely.
We had ALOT of issues with the duty solenoid(in rear of trans), remove ext housing and viscous coupling/transfer gear and you will see a small solenoid in the ext housing, make sure you order two gaskets first, you will see there is a steel plunger in an alloy housing with oil galleries in it. This is the valve that controls front to rear drive. Remove the steel plate that holds the sring in, (careful not to lose the spring!!), remove the spring and the plunger should "fall out under it's own weight", I can promise you that it won't, it will need to be removed gently without damage, then run it down with some 1200 of finer sand paper, wrap some paper around a screwdriver or simillar, and do the same to the sleeve/bore that the plunger fits into,(taking care not to damage the bore!). Make sure the valve will 'slide up and down' without the spring in it by shaking the housing, you should be able to hear it click when it moves. Then clean it all down with white spirit or simillar. Slap it back together and fill er up!!
Don't forget to mark your tailshaft for re-assembly to avoid vibrations. Make sure to order a new Ext housing gasket-if fitted. Make sure ALL clutch plates are aligned correctly with 'drive-drum' prior to re-torquing ext housing bolts.
Loctite t/shaft bolts. PHEW!!!
I thunk I straiyned miy fungers!!!

Cheers

Sixpack