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This is most likely 2 problems. The ABS/Brake light and speedo issue will be the speed sensor located in the read differential housing. The temp guage issue (and I'm assuming this is at idle) could be thermostat or temp sending unit. The fluctuation is causes by the electric cooling fan cycling on and off to control the temp. However, most of the time this fluctuation should be from about 195 - 220 degrees.
It is much more likely you are low on coolant or you have an air bubble trapped in the cooling system. You should bleed the cooling system to get any trapped air out and make sure it is full of coolant.
I've had problems with Datsun (later Nissan) engines hanging the closed thermostat until it boiled, then the thermostat released and operated normally. If you increase the small vent hole in the thermostat valve plate a small amount, the problem disappears. It seems that steam from engine operation gathers behind the thermostat sensing bulb and delays opening. The increase in vent hole size lets the steam through and allows the hot water to open the valve. Hope this helps!
Sounds like the fan and temp gauge (located on the intake mainfold is screwing up) , if its head gaskets it would be smoking out the tail pipe. Have you added any antifreeze if so was it once a week if so make sure nothings leaking on the ground if there isn't then its your head gaskets that would be a first for a 4.6 which is a good motor and none I would on ever had bad heads. I would check the water pump fan first with truck off grap fan at 9 and 3 and see if it wobbels forward and backward if so it's the fan. I have come across alot of bad fans. Fan would cost $30 and up. Rate of difficulty 1-10 I give it a 4.
sounds like you may be low on antifreeze. ck and top off as nec. then run vehicle again if was low. if engine is running at normal temp (around 195-200 degrees) and not fluctuating, then a possible restricted heater core is suspect. you can flush these cores out fairly easily at firewall under hood. (do this before replacing core, pressure washer works well). if engine temp is not normal(reads low) or fluctuates, replace thermostat.