Recently it feels like the road has "expansion joints" every 3-4 feet - bumps really bad with a steady rhythm. The shop said front driveshaft was bad, so we removed the front driveshaft and ran for a couple of weeks with no problems. We have since replaced the front drive shaft with a new one. It ran perfect for about 3 weeks but now is back worse than ever. I am afraid to drive it for fear of tearing something else up. Any suggestions?

I'd try taking the front driveshaft back off, change the transfer case fluid to the blue AutoTrac II fluid available at a local Chevy or GMC dealer, drive it a few hundred miles then re-install the front driveshaft.

What transfer case does it have?
Are you getting any check engine lights.
Does the service 4W come on for a bulb check and then go off?
All all tires and wheels matched/same size and height?

Not sure about the transfer case but we don't get any check engine lights and will check for the 4WD light. We did put new tires on the front just before the problem started. They are different brand but supposed to be the same size and height. Could that be the whole problem?

Could be .
Check for codes and post back codes and what transfer case you have.
Has the transfer case ever been serviced?

Not sure if the trasnfer case has been serviced. We have checked for codes - nothing. Will try to check what transfer case it has.

Not sure about the transfer case but we don't get any check engine lights and will check for the 4WD light. We did put new tires on the front just before the problem started. They are different brand but supposed to be the same size and height. Could that be the whole problem?

You might rotate one front tire to rear and one rear to front and see if it helps.
Also depending on what transfer case it may need the service done with the GM auto trac fluid.
MT

Should the new tires be on the same side?

Not sure about the transfer case but we don't get any check engine lights and will check for the 4WD light. We did put new tires on the front just before the problem started. They are different brand but supposed to be the same size and height. Could that be the whole problem?

Using different brand of tires, even though they are the same size, could cause a problem. Even though they are the same size, they could still be different in design, and in turn, have different rotational speeds and could cause a problem.
I also agree with Chris Stewart, MT, take a look at the fluid in your transfercase. I would change the fluid in it, but make sure the proper GM autotrac fluid is used.

The 98 Bravada has the New Venture Gear 136 transfer case. The tires are a very real possibilty. When replacing tires on one you must replace all four tires at the same time, with a matched set. As stated, having different brands can cause problems. Changing only 2 at a time causes huge problems, because they will not be the same size as the other two due to normal wear. The old tires have less thread depth due to wear, causing them to have a smaller circumference and turn at a different speed. In an AWD this difference in speed causes more stress on the transfer case clutch resulting in more friction and heat, which degrades the fluid. It also causes the drivshafts to rotate at different speeds, which is picked up by the front and rear speed sensors on the transfer case, fooling the TCCM into thinking that one of the axles has a loss of traction. A difference of only a couple milimeters in tread depth can cause malfunctions, damage, and accelerated degradation of the fluid. Also as stated above, when servicing your transfer case, you must use the blue Autotrac II fluid available from your local dealer.

Thanks guys. Got two matching tires ordered. Finally got my truck back!