I just bought a 4.10 from a wrecking yard because I have the same top end problem. Be sure you check ratios before pulling it. Not all '67-72 automatic 3/4 tons have the 4.10. I went through 3 before I found one-the others had 4.57's like the one I'm replacing. You can find the numbers on the pinion shaft-along with dates etc you will see something like "7 32" which means 7 pinion teeth and 32 ring gear teeth. 32/7 = a 4.57 rear end. The donor I ended up with is 41/10 or 4.10 ratio. I'm currently trying to locate seals so the thing won't leak.