Pretty sure there's a spline count difference on the first couple years of gmt400's too, so the '88 was not a good donor for your '94. It's been forever since it mattered to me to remember, but I'm about 99% sure that '88-'89 use a 28 spline axle, and '90-up use 30 spline.
As others have mentioned the 2WD and 4X4 use different width axles, the 4X4 is wider. No telling what was in the truck you removed the axle shaft from, if it's shorter than yours, it's possible somewhere along the line someone put a complete 2WD rear end in the truck when their original failed. 10 bolts don't last forever, many of them have been swapped since these trucks began getting old. I used to sell used 10 bolts all the time when I ran my salvage yard more than a decade ago.
Richard