After replacing universal joints and center bearing vehicle started shaking at 15 MPH and stops at 25-35 MPH

When this starts shaking, does it stop if you remove your foot from the accelerator?

More than likely the driveshaft is out of balance. This happens when they get older. A lot of times they have weights welded on them and the weight will actually fall off. Then it starts to vibrate and if you remove it and don't index it, meaning mark the spots that it was bolted to each flange so that goes back on the same way, the vibration will be worse.

What I would do is remove the driveshaft and take it to a drive shaft shop and have it balanced.

https://www.2carpros.com/articles/how-to-remove-a-drive-shaft

Otherwise, you can try doing the hose clamp trick which is use a hose clamp and attach a small wheel weight to the driver shaft in front of the rear differential. Then drive it. If it is worse, stop and rotate the weight around the driveshaft until you find the best location.

Clearly this is time consuming and not a guarantee that it will solve it. The best way is to have the drive shaft balanced and then you can actually index it on the vehicle which will balance it while it is spinning on the vehicle as each of the flange will be slightly distorted.

Most balance shops will do this as well but it may not be needed if the drive shaft is out of balance.

Please let us know if you have other questions on this.

Thanks