So i just bought an 88 suburban, and the only way to drive it is in "4" wheel drive, where only the front wheels engage. Any ideas whats wrong? Also, what transmission does this beast take?

 

In the case of a 4x4 the rear axle rod does not connect to the transmission, instead it connects to a transfer case. So if the rear wheels are not engaging, there is likely a problem with that, and not the transmission.
For that year model the transmission is likely the, older Turbo 400 (3 speed), or the 4L60 (over drive)
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Thanks for the reply
Ok, i think i found the issue. When i have it in RWD, and put it in drive, the driveshaft spins like crazy. This leads me to believe its a problem with the rear end/differential. Know what rear end goes in this?