Best would be to find another comparable truck with the gearing you want, and swap the front and rear differential out. You can change seals, pull the rear cover and check out the bearings and internals before bolting into your truck. Changing your current gearsets out would be a lot more costly.
With the difference in cost, you can buy a new rear axle cover, u-bolts, backing plates ( I would recommend new ones ) as well as fresh brake parts, wheel cylinders, shoes, hardware kit, drums, e-brake cables, etc.
Also, if you have the 10 bolt rear axle, it may be worth it to find a SF 14 bolt to hang in place of the 10 bolt. You would need new brake cables if going this route anyways, they are different lengths.
These trucks are plentiful in scrapyards by now, the one close to me has 40-50 at any given time.