Sounds like the Rear end is whining. On load whinning (accelerating) with the whining going away when coasting or de-accelerating is indicative of pinion and ring gear mis alignment, or worn pinion gear bearing/shims. My 95 Jimmy whined like this from almost day 1 all the way up to 126,000 when the bearing finally let go. Find another S-10, Sonoma, Jimmy, Blazer or Bravada with the same gear ratio and swap it out. Install NEW U-BOLTS with the Axle.
2002 S-10 ZR-5 Quad-CAB
1995 GMC Jimmy 4Dr. SLT