axle seals aren't too hard to change, you need to pull the tire, pull the brake drum, remove the differential cover, get into the carrier and push the axle shaft in to allow removal of the C clip (there should be an access point somewhere on the carrier, different carriers are different on how they are set up though, I'm use to my detroit which is different that a GM carrier so I can't remember how GM does it, I think there is a cross shaft bolt that needs to be taken out before you can get to the C clips.)....then once the C clip is out, carefully pull the axle shaft out, pull the old axle seal, and tap a new one in, then reverse the process to re-assemble everything. its really not too bad.
as far as fluid goes, I usually run either 75w-90 or 80w-90. I prefer synthetic everywhere. there used to be a time when synth could cause leaks due to higher detergent amounts, but that was back 30 years ago, synthetic oil technology has come along alot. switching to synth shouldn't cause any issues whatsoever. and it has higher sheer strength and breakdown resistance (among other things) than conventional. however it does cost more, so if you can't justify the cost, there's nothing wrong with conventional. whatever you do though, get a quality brand. I prefer either M1 or RP for the gear oil...in fact I make my own concoction for the rear end, I mix some ultra fine molybdenum disulfide in with the gear oil to help further reduce friction and the subsequent heat in there. works great.