Fuel gauge pegged way past full

The common reason for that is either a bad sending unit in the tank or the gauge itself is bad. GM gauges in 1995 used 0 ohms for empty (ground) and about 90 ohms for full.
Another cause is a broken ground to the sender or tank. However, if your truck runs that ground is good.

The "easy" way to figure out which is with an OBDI compatible scan tool. It can read the tank sensor and tell you if the tank says it is quarter tank (or whatever level) and the dash is pegged it is the dash gauge. Or if you know you have about half tank and the sender reads 100% you know it is the sender.

The "hard" way is to go under the truck, locate the wiring harness that goes to the tank and sender. Find the purple wire and disconnect it. Then touch it to a good ground. If the gauge drops to empty the sender is likely the problem. If it does not move the gauge or dash is the likely issue.