[Ski-me]

The only way to tell for sure is to pull the diff cover off and see what's stamped on the ring gear. Might be worth changing the diff fluid anyways.

Stamping:
41:11 - 3.73 gears
41:10 - 4.10 gears
41:12 - 3.42 gears

These trucks aren't known for a lot of strong power, especially trying to hold 80 MPH. With the 4" lift, you're pushing a LOT of wind at that speed. With the 4 speed auto, I almost always downshift when I hit the gas on the freeway.

Changing gears really can get expensive, so usually not worth it. Does it feel pretty quick from a standstill? If so, that might imply a lower gearing axle....and if your speedo is reading higher than actual, also points to lower gears a bit. OR, maybe someone tried to just change the speedo gearing rings at the t-case.

3.42's and 33's at highway speeds are not going to win any races.

Great looking truck though! How's the interior and underside of truck? WI rust?