If your not leaking tranny fluid i wouldnt see why you should pull it all apart to place new seals in it... unless you were only gonna do the seal in the tail shaft? If the joints in the drive shaft are good and tires arent your issue, i would look at mounts, rear end, and possible timing. Im a test everything you can think of kinda guy. It results in issues being fixed and lots of new stuff on my old beater