It's my understanding that any axles from our era on up to the late 80's/early 90's (with the caveat that they're from a similar grade vehicle) will be more or less a bolt in swap. I wouldn't call myself an expert on it by any means, but anything from the mid 70's on up would still fit the truck and give you better choices for gearing. Be sure to double check the track width on any axle you're thinking about using. You also may need new U-bolts to mount the axles. I don't remember the name of the company, but I had a firm in Canada make me new bolts to replace the originals. The front axle from the '88 had the same tube diameter, but the differential casing was forged a little differently and required a change in U-bolts. A minor setback, but still something to be mindful of. Usually if you pick axles from the eras I mentioned above from similar grade vehicles you won't have much trouble with the swap. Just be prepared to change the spring perches in the rear and perhaps some of the U-bolts up front. Hope this helps!
Matt
