so i have a 1990 fullsize i just bought with auto 4x4, the previous owner stated that he thinks the 4x4 doesnt work, i checked the case and it sends power to the front diff, havent had a chance to try to engage it though, anyway here the problem, whhile in 2wd under heavy acceleration occasionally, and at other random points i rear like a really loud bang/pop, scares me and im a mechanic, i just dont have much experiance with the 4x4 on these truck and am not sure if its related, any ideas? i do not belive it to be the rear dive shaft cause the ujoints are new

 

pull the drive shaft and dab a little bit of anti-sieze on the splines and reinstall