94 Sub Rear Shocks /Susp

I have a 94 C2500 Sub. I've owned it for 8 months, Have been working on it for 3 years(I bought it from my boss) A year or so ago the upper shock mount broke out of the cross member on the left side rear. Since the top mount was part of the rear crossmember/body mount, I was able to repair it by grafting in a piece of a mount from a pickup. Big question! Why did it break in the first place? I ordered a new set of rear shocks(Delco)the part # matched the ones that came off. When installing the shocks I noticed that there was very little travel remaining after installed. Less travel than bumper to axle distance. I also found that Installed length of the shocks was different(over 2 inches longer on the right side). I double checked the part# on the shocks with the dealer, they assured me they were right. I took some ride heigth measurement at the rear of the truck and they were within spec, ruling out bad rear springs. As well it sits at a normal angle. The spring eye bushings are good, so the axle isn't moving backwards under spring compression(the left shock mounts from the axle towards the rear bumper). The truck sits tall but that would give the stock shocks more travel.
Long story short I hit a good side bump today and the top left rear shock mount snapped again. I'm sure that the shock bottomed out. Hopefully I won't have to replace the crossmember this time. The only modification to this truck is rims.
AC Delco's website shows 2 part#s 530-238 (javascript:displayDetails( 'ACDELCO', '09', '530-238')),520-61 (javascript:displayDetails( 'ACDELCO', '09', '520-61')). The # on the shock is737985. I could not cross that # on Delcos site. Could they have sold me shocks for a 4WD? I'm far from new to this stuff. I have over 25 years in the buisiness with 6 years doing large truck frames and suspensions. Any ideas?
Sorry for long post just wanted to give as much info as I could. Thanks in advance for any ideas. Billy