The tachometer is showing 5000rpms and the engine is off. When driving down the road it is past 7000 rpms, but the engine does not appear to be racing and the speed is 55

vacuum leak usually increases rpm--the electrics got hurt from overheat--hav a data stream run-displays values of evry component-so u kno what parts are bad----private mechanic not a dealer- a scanner that uses factory gear not a 30$ tool sold in stores--the vehicle computer may hav an issue-a readout wont disclose it--so disconnect batt n see if problem clears or pull plug on computer and clean the pins--may reboot

Sounds like a dirty air filter is putting the mass airflow sensor out of range. The throttle position sensor is subject to physical wear, the age of your truck warrants replacing it. The coolant temp sensor controls the Idle Air Solenoid which determines idle speed ( cold, high idle, 1200 rpms open loop mode for the ECU) and (hot, low idle, 800 rpms, closed loop mode for the ECU) If the truck does not have a Mass airflow sensor, check the Manifold absolute pressure sensor (MAP). Perform an OBDII scan for stored fault history to avoid buying parts you don't need. Do the cheapest fixes first. Go ahead and change the fuel filter also. There is a lot of water in todays fuels

Your probably going to have to bring the car for service. When it is idling and reading 0 can you rev the engine? Does doing this change the tachometer reading or no? When you drive it is it only going to 1000 rpms maximum? If so, it seems that you aren't getting to the higher gears. Either way, if it continues, definitely bring it in for service.

i had a 2001 aurora with the same exact problem. lucky for me a drunk driver smashed into it with a plow truck and it tumbled down a cliff into a river while it was parked and they gave me 12k for the car that i paid 8900 for .
i beilive its and ignition problem

you did replace computer also, the engine you put in will need the computer for it,also get all connections.

First I need to ask is it a 4 speed or a 3 speed transmission. If it's a 4 speed when driving down the road pull it from drive to third gear. You shpuld hear the engine speed up if it does not then that means you have on 4th gear and you will have to take it to a transmission shop. If it's a 3 speed then just count the shift points if all 3 gears are present then drive down the road and see if the van goes into tcc. When you mean the rpms are high how high are they.

get a moto vac cleaning done on engine that will clean all injectors and test fuel pressure should cost 100 or a little more fuel savings will pay it back in no time moto- vac cleaning

if its an automatic the trannys going out if its a manual the clutch plates could be going out

you may have a vac.hose leaking some were on the motor

this sounds very much like a torque converter/transmission issue. i will assume the transmission dial you're speaking of is the tachometer? which measures engine revolutions per minute. to confirm, one could possibly unplug 4-wire connector on transmission near radiator and test drive. this keeps the transmission from going into torque converter lock-up.