I had the same problem on my 84 CJ-7. However I opted to change out the gauges. I cut new holes in the dash and put regular gauges in. When I did this everything worked. I also tried switching speedo assemblies before this just to find that the gauges were bad in it too.
Also I was reading up in 4 wheel drive's catalog and they recommend changing both the fuel and temp gauges as a pair. This of couse in your case will be nessecary. these gauges aren't very expensive adn can be picked up from 4 Wheel Drive Hardware
. Hope this helps.
'84 CJ-7 Junkyard TBI 360/T18/D300, 4" 4WDH Lift, D44 front with ARB and HS/D44 Rear w/ Detroit, 4.56s, 36" Iroks, TFI, TJ flares, Homemade rocker guards and bumpers with swing out, Optima Yellow Top, M8000, York OBA, 6pt cage
"when you're up to your *** in alligators, it is hard to remember that your original intention is to drain the swamp"