Are or could kangaroos be a distant evolutionary cousin of the T Rex?

Every living thing on earth is a distant evolutionary cousin of any other organism.

The 2 groups of your example, are indeed very distant. 

Their (and our) last common ancestor dates back to around 312 million years when synapsids (mammal ancestors) split from other amniotes (reptiles...).

Anyway, whether they were mammal, turtle, dinosaur, or bird ancestors, all Carboniferous amniotes look roughly like a modern lizard

they could if your writing a book and you need them to be...but they are probably not irl cause one is a rodent and one is a lizard

T. rex is an archosaurian reptile and crocodilians are living archosaurian reptiles. The ancestors of birds was also an archosaurian reptile.  That means T. rex is actually more closely related to crocodilians and to birds than it is to lizards and snakes.  Lizards and snakes are diapsid reptiles but not archosaurian reptiles, Archosaurian reptiles are diapsids. Kangaroos are mammmals, and that means kangaroos are more close related to bats, rats, cats, pigs, whales, monkeys, seals, humans, horses, cows and koalas than they are to T. rex. 

Although T. rex looks sort of like a kangaroo, looks can be deceiving. Animals look certain ways because they evolved to live their particular ways of life. Whales for example look like fishes because they live in the same environment as fishes, and the fish shape is what works for animals that live in the water. Scientists call similarities between animals that are not close relatives but that nevertheless resemble each other convergent similarities.  

In that everything alive is a descendent of the Last Universal Common Ancestor, yes, kangaroos are related to T. rex. 

However, there is no way that kangaroos are a direct descendent of T. rex. Kangaroos are mammals and mammals are not descended from T-rex.