Re: 1972 J-4000 TRUCK
That truck, if it is stock, should have a Dana 44F or Dana 44 Heavy Duty (HD) front (8000lbs GVW only), and either a Dana 44 HD or Dana 60 one-ton rear. The 7th digit in the V.I.N. will tell you what the Gross Vehicle Weight (GVW) is:
W = 5000lbs = Dana 44HD
X = 6000lbs = Dana 44HD or Dana 60-2 (optional semi-float)
Y = 7000lbs = Dana 60-2 (semi-float)
Z = 8000lbs = Dana 60-3 (full-float)
The front axle is the same for all J-trucks 1970-1982, and the rears are the same 1970-1973 ONLY! In 1974, AMC changed the rear axle to the Dana 60 or 60FF with the 8-lug pattern.