Since $1998 = 37 \times 54 = 74 \times 27$,

(1) By the pigeonhole principle:
Among the 93 distinct positive integers $a_1, a_2, \ldots, a_{93}$, there must be two numbers whose remainders are the same when divided by 37. Let these two numbers be $a_m$ and $a_n$, then $a_m - a_n$ is a multiple of 37;
Among the remaining 91 numbers, there must be two numbers whose remainders are the same when divided by 54. Let these two numbers be $a_p$ and $a_q$, then $a_p - a_q$ is a multiple of 54.
Therefore, there must exist four positive integers $a_m, a_n, a_p, a_q$ such that $(a_m - a_n)(a_p - a_q)$ is a multiple of 1998.

(2) By the pigeonhole principle:
Among the 93 distinct positive integers $a_1, a_2, \ldots, a_{93}$, there must be two numbers whose remainders are the same when divided by 74. Let these two numbers be $a_m$ and $a_n$, then $a_m - a_n$ is a multiple of 74;
Among the remaining 91 numbers, there must be two numbers whose remainders are the same when divided by 27. Let these two numbers be $a_p$ and $a_q$, then $a_p - a_q$ is a multiple of 27.
Therefore, there must exist four positive integers $a_m, a_n, a_p, a_q$ such that $(a_m - a_n)(a_p - a_q)$ is a multiple of 1998.

In conclusion, there must exist four positive integers $a_m, a_n, a_p, a_q$ such that $(a_m - a_n)(a_p - a_q)$ is a multiple of 1998. $\boxed{\text{Proven}}$