You can replace the pinion seal by unbolting the driveshaft, and then the pinion flange . Use a seal puller or something to pry the old seal out, then replace with a new one and reassemble.
I do belive there is more to it than that though. How long have the gears/bearings been installed in the vehicle? It is likely, if everything moves around like you say, that the pinion bearings are shot, or the pinion nut has backed off.
Take the driveshaft off and check to see if the pinion nut is staked into the small notch in the pinion. If The flange is held tightly to the pinion move the flange/pinion front to back. If it moves excessively then you are in need of a rebuild, and possibly new r&p gears.
As DTB said nice rig, keep the 36s. 5.29 should be enough gear for them.
