Hmm...
The suburban you have likely has the 700r4 overdrive transmission right (its the only overdrive tranny that was ever installed from the factory in the older subs minus a rare standard overdrive transmission)
The 700r4 is known to be notoriously weak, and breaks often. But before you give up all hope, make sure that all your vaccum lines are hooked up, and make sure that your motor is making enough vaccum to actually be able to run the transmission. They shift depending on the amount of vaccum they've got. So check to see if that steel line has been broken off, torqued, or if the rubber seals on the line are rotted out and not holding any vac anymore. Another possibility is that the little shifting linkage on the steering column can occassionally get messed up and not accurately show what gear you are in. When you shift into drive, count the number of clunks you hear to make sure that you are infact in drive, and not in first or something.
If all this fails, you may be out to find yourself a new transmission. Definitly a fun swap to say the least, but far from impossible if you have some simple tools, and some time to work on the burb. If you don't have a 700r4 tranny, this may be a good time to find one, get it built up a little bit (they can be built to withstand up to 700 hp now using corvette servos etc) and get it installed. The overdrive would help you wouth with your gas milage immensely! If you are planning on doing a lot of offroading, and generally like beating on your transmission, you'll want to get GM's heavy duty transmission, the TH400, its nearly bulletproof, and it will take a real beating before it breaks. All these transmissions can be found in your local wrecking yard, and often they will help you pull it out of the donor vehicle, if not do it for you at a certain fee.
Now, since you have a 4x4, you'll have to make sure that you get a 4x4 transmission -- the ones from a 2wd won't fit properly, you'll also want to make sure that if you get a 700r4 (and don't already have one), that you get one of the ones that doesn't require a computer to operate, just vaccum, and a manual lockup that you can install on a switch inside of your burb. I think that it is the pre 85 700r4s that you want (don't hold me to that though, I'm not 100% sure about it...) If you already have a 700r4, make sure that you get one with all the same connections as your current one already has.
Changing it out is a matter of either pulling the whole engine out, or removing your driveshafts, unbolting the tranny, and pushing it backwards a bit with a jack under both it and your motor, then lowering the jack, and putting the new one into place. There are several members here who have done this swap recently, and I'm sure that many would be plenty willing to give you some pointers if you need them.
I personally would just pull the whole engine, but being an 88, you most likely have a LOT of pollution controls that my 80 pickup doesn't have.
I hope this helps you out a little bit, and I hope that I am right in everything that I said. If not, I'm sure that there will be someone who corrects me and sets things striaght.
BTW -- Welcome to the Chevy / GMC truck forum, there are some really really wicked rides here, some that have been featured in magazines, down to some kids like myself who are trying to make something nice out of their old beaters
