I'll go out on a limb and against what everyone else is likely to tell you, but hey, I got balls so I'll take the heat. In all honesty, it's true the Gov-Lok is a hunk of ****. But, I've seen an 80 GMC 1/2 ton 12 bolt with one that worked perfect just before I sold the rear end a few years ago and who knows many thousands of miles were on it. I bought it from an old man who bought the truck new. I've seen it in 2 different Full Floaters and it was in good shape and worked well. No, it's not the most favorable of Lockers/LSD units, BUT for a farm truck and what you mention you're going to use the truck for, I'd go with it and leave it alone. If you're truck is an 81 and it's worked all these years and it's still working.....if it ain't broke don't fix it. Know whutta mean??? And I expect it'll work out just fine for you on the farm. So save your money and keep what you got.
If you want to test it, get the truck on some level concrete. Jack up one tire on the rear end with a floor jack and make sure it's off the ground about 2-3 inches. Foot on the brake, in 2hi, but it in D and slowly let off the brake then give it a little gas and drive the truck on the floor jack for a few feet. You can expect to hear a clunk when it locks up just before it takes off moving. If manual trans, just slowly let the clutch out like your trying to take off and drive it on the jack. If it moves 4-5 feet or at all actually, you know it's working. Reason I say slowly let off the brake and ease into the gas, or slowly let the clutch out, you don't want to slam it and damage the unit.
Of course you can also test the regular way just finding some slick dirt, gravel, mud or snow and give it hell and see what happens.
Don't launch it. Just go easy with that test on the floor jack. It's normal for it to spin half a turn or so, you just don't want it to slam when it grabs the other axle. That's how you can break the governor lock or maybe even strip teeth on the wheel.
