Fuel gauge only going to 3/4 when grounded?

You need to treat this as an END TO END problem instead of thinking about "gauge" etc

THINK about the ENTIRE PATH from power to the IVR through the various connections, gauge, wiring, terminals and to the sender

If you can get the cluster out, "rig" it to the proper 12V terminal and test it with a known test resistance. These have been published on this board hundreds of times

But grounding the sender wire and only getting 3/4 is for sure a bad sign.

I would pull the cluster and test it. If it still has a problem, you just eliminated the entire sender wire/ harness/ power source

Test the OTHER GAUGES in the cluster to be sure they don't do the same thing!!! Maybe the IVR or it's connections are causing low power to ALL gauges. (I don't know if you have oil and temp both)

Since you have it out I would at minimum do the following:

1...harness connector pins are and can be problematic on the older cars, don't know about trucks. Check them, solder them if necessary

2....Same with IVR connector fingers. Solder jumpers from connector fingers to board traces or wherever they go. Again, I'm not really familiar with the later trucks

3...Strongly consider replacing the IVR with a modern solid state one

4....RE-test any gauges supplied by IVR. If all three, they all have same characteristics, IE the same test resistor on any of the three should give same readings on both/ all three gauges.

5...Add a ground pigtail to the cluster at a common ground point, and bolt that to the dash frame/ column support